How I prevented QRM to HF reception from my solar charger and AC inverter at Quartzfest

Figure 1:
White board from Quartzfest!
Click on the image for a larger version

As it happens, I found myself at QuartzFest in Arizona in the latter half of January, 2026 where we set up some banners proclaiming the existence of the Northern Utah WebSDR (link) - but I also scribbled on a small white board the words "QRM-Free Solar is possible - Ask How!".

Between the SDR, this message and the diverse portable HF antennas erected, I have had a lot of conversations over the past several days about these and many other topics, meeting new people and re-acquainting myself with others that I've seen on and off over the past several years of my attending QuartzFest (this is year #4 for me.)

RFI-less solar IS possible 

During the "Solar Walkabout" - an on-foot tour to look at how others camping have deployed their solar panels - I volunteered to have folks look at what I'd set up:  It's nothing obviously special - a glass-panel 200 watt Renogy folding array and another Renogy "flexible" solar array - but there is one major difference:  It does NOT produce HF QRM, meaning that I can plant my portable antennas near my panels and not get any interference on HF.

As I've done some previous articles on this, what I'll present here is mostly a set of links to those articles with a quick overview, but this effectively puts that information in one, handy place.

Let's start with quieting the Renogy solar charge controllers:

Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system- Link

Figure 2:
My humble, RF-quiet solar array at 2026 Quartzfest
Click on the image for a larger version.

The main issue with Solar charge controllers is that you have a "dipole + transmitter" situation:  The panels themselves do NOT cause RFI, but the charge controller is effectively a transmitter - especially if it's a PWM and/or MPPT-type - and the legs of the "dipole" are the solar panels (possibly long wires connected to large, rectangular pieces of metal) and another set of wires going to the battery - which also find their way around your RV/campsite via the inverters, DC wires, etc.:  It is no surprise at all that RF finds its way out of these things!  By adding filtering, we are effectively "shorting out" the the RF at the feedpoint of this hypothetical dipole and preventing it from radiating.

To quiet these panels, I added bifilar-wound ferrite toroids - but also bypass capacitors:  The toroids (ferrite) alone will probably knock down the QRM by 2-3 "S" Units, but if you are getting S-9+ interference from your solar, simply knocking it down to S-6 or S-7 when you are in the boondocks - where the natural noise floor is closer to S-1 or S-1 - is still pretty bad!

The key here is adding capacitors in addition to the ferrites and this method is perfectly capable of quieting even the noisiest of solar chargers.  It is also vitally important to put this filtering physically close to the noisy device and use good-quality bypass capacitors. 

Figure 3:
Filtering on the bottom of the Renogy controller
making it RF-quiet.
Click on the image for a larger version.

While the above blog entry showed a modest (200 watt) system, the above can be scaled up for higher-power systems:  Larger wire will handle more current and larger toroids will accommodate it!

RF Quieting a Samlex 150 watt Sine Wave inverter - Link 

Another component of RV/camping with power is the inverter to run mains-voltage devices, and these can be terrible noise sources.  The article above shows how it's possible to make one of these devices completely quiet.  For the older Samlex inverter - which was terribly noisy out-of-the-box, it is now quiet enough that I can power LED Christmas lights from it that are strong from the same mast as the antenna and I get NO RFI (the LED Christmas themselves don't produce QRM).

I was fortunate that there was enough room in the Samlex's case to be able to add this filtering, but it may be added externally as well, provided that the leads are kept short.

What follows below are some methods for quieting UPSs (Uninterruptable Power Supplies).  These are very much like the inverters in an RV in that they produce mains voltage from battery power - and the same problems with RFI occur:

A high-current DC (and AC) noise filter for UPS or RV use - Link

This shows a rather extreme example (an 8kVA UPS) where high currents are involved:  Such would be the case with a kilowatt-class DC-AC inverter or even a large PV system.

Containing RF noise from a sine wave UPS - Link

This article shows the techniques involved in quieting a lower-power UPS, but it also introduces some other components:  Rather than winding your own filter using toroids and wire, you can get "Line Filter" modules from electronic parts supplies (e.g. Digi-Key, Mouser) with brand names like "Corcom" or "Delta" (among many others.)  These are self-contained modules with the components built-in - available in a wide variety of voltage and current ratings - that can do an excellent job of filtering.

 

Completely containing switching power supply RFI - Link

This is an extreme example, but it shows how one might be able to make even the noisiest switching power supply quiet - and this might be important to someone who is trying to get every device in their ham shack - whether it be at home or on the road - quiet.  This method is foolproof in its effectiveness, but it is also likely overkill for many applications, but it discusses the "how and why" these techniques can work.

* * * * * * 

I hope that this helps those who venture out in the wild with their RVs, solar power and battery system and still be able to operate HF.

This page stolen from ka7oei.blogspot.com

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Analysis of interference from a SolarEdge PV (solar) electric system.

Comment:

This article - while it centers about the investigation of a SolarEdge PV (PhotoVoltaic) system - the discussions of techniques and strategies should be generally useful when investigating interference from any make or model of PV system - or even interference from other sources.

* * *

Follow-up:

There is a follow-up on this article where I note that the signals emitted by multiple systems can be propagated over great distances via the ionosphere - link HERE. 

Several months ago I got a call from a local amateur who was very concerned about a sudden rise in his noise floor across the HF spectrum (3-30 MHz).  This increase in noise seemed to be coincident with the installation and commission of a 5 kW PV (Photovoltaic, or "Solar") electrical system on the house of an adjacent neighbor.  I suggested that he talk to the manufacturer of the PV system to discuss the situation - and to request from them possible solutions.

A few weeks ago, he got back to me and he had, in fact, talked to the manufacturer and an online meeting was arranged in which they would remotely idle the neighbor's system while we were monitoring via the amateur's receive antenna.

Out of curiosity - and as sort of a practice run - I went over the weekend before the online meeting to get a better idea as to the spectral signature of this system - a SolarEdge series string system with optimizers - when it was operating normally.  The amateur had obtained permission from the neighbor to allow us to enter their yard to make very "close in" measurements (e.g. within a few inches/cm of the equipment, conductors) to obtain spectral "samples" of the system, thereby excluding external signals.

For these measurements, I used an amplified, shielded magnetic ("H") loop antenna (about 18"/50cm) in diameter as the "sense" antenna and an HP/Agilent/Keysight spectrum analyzer, recording the plots electronically - although a Tiny SA "Ultra" would likely have sufficed as well.  None of the readings were to represent "absolute" signal levels as all that we were really interested in were relative measurements, and as such all that we needed to do was keep our measurements consistent - that is, being able to precisely repeat the conditions between subsequent measurements.  These readings would allow us to understand the nature of the RF energy that it was creating - its "signature", if you will.

Note:  For information about the H-field loop used for this testing - and using an inexpensive spectrum analyzer such as the "Tiny SA" or, better yet, the "Tiny SA Ultra", see a previous blog entry "RFI Sleuthing with the Tiny SA" - Link.

The nature of this QRM:

The interference observed by this amateur - now known for certain to be signature of this model of SolarEdge PV system - was evident as two general types of signals:

• Moderate to strong clusters of carriers every "even" 200 kHz.  At 200 kHz intervals (e.g. 7.0, 7.2, 7.4 and 14.0, 14.2 and 14.4 MHz) from below 3 MHz through at least 30 MHz could be heard a melange of closely-spaced carriers within about 500 Hz of each other on the lower bands.  While these carriers sounded like mostly CW (unmodulated) signals, there was also evidence of low rate data signalling buried in the cacophony as well as additional lower-level carriers.  (The actual interval is very slightly smaller than 200 kHz so these "bunches" of carriers will be a centered a few hundred Hz below exact, even 200kHz multiples, the difference increasing with frequency.)
  

• Background "white" noise amplitude-modulated at the mains frequency.  If you were just casually listening at this amateur's QTH on the HF bands - say 40 meters - you might be forgiven in the short term for presuming that nothing was wrong.  In reality the noise floor had been elevated several "S" units by the PV system - the result sounding superficially like plain, old white noise:  Switching from SSB to AM reveals the loud "hum" that is riding on the noise - modulation that is almost "invisible" if one is listening only using SSB.

While the appearance of the above interference coincided with the activation of the neighbor's system, that fact that it disappeared at night further pointed to a PV system as the source of the QRM.

"Close-in" measurements

Placing the sense antenna right at the main inverter on the back of the neighbor's house, we wanted to take a snapshot of the spectrum at that location:

Figure 1:  0-30 MHz sampled right at the main inverter

With each horizontal division representing 3 MHz, this 0-30 MHz plot shows a high concentration of noise in the 3-9 MHz area from a location right at the inverter.

Because Figure 1 represents the spectrum at the inverter, we wondered what it would look like at one of the solar panels so we placed the sense antenna right against one of the solar panels:

Figure 2:  0-30 MHz sweep with sense antenna placed next to a solar panel

Figure 2 is in the same frequency and amplitude scale as Figure 1 - but with the "reference level" adjusted by 20 dB to move the trace "up" a bit - and we can see that the spectrum next to the panel looks quite different from that sampled right at the inverter.  This isn't unexpected as Figure 2 would likely represent more of the noise that is emitted from the DC (input) side of the optimizer whereas the spectrum represented in Figure 1 would be more likely to show that of the DC output of the optimizer plus whatever noise was riding on the conductors carrying the DC input and AC output of the main string inverter.

Although it is difficult to be sure, the 0-30 MHz plots taken from a greater distance (10 meters or more) had the general appearance of the noise spectra shown in Figure 2 more than that of Figure 1 leading me to believe that a significant portion of the QRM may be being radiated from the panels themselves rather than just the conductors going from the optimizers  to the main inverter - but certainly, both are likely involved.

Note:  For both of these plots, the RF energy from the PV system was many 10s of dB above the typical background noise floor - in this case, 40-50dB for Figure 1 and at least 30dB for Figure 2 in the area around 7 MHz.

As the 0-30 MHz sweep does not have enough resolution to visualize the narrower 200 kHz signals, the analyzer was readjusted as depicted in Figure 3 - again with the sense antenna next to the panel:

Figure 3:  From near the panel, a "zoomed in" spectral sweep showing narrowband birdies.

In this spectrum plot we can see not only the "white" noise on the floor of the sweep representing the "hummy hiss", but also the much stronger signals every 200 kHz - plus a number of weaker signals in between:  It is these signals that are the most obvious to the casual operator and appear to be unique to a SolarEdge system of this model/type.

On this same day we waited until after sunset - monitoring the groups of carriers at 7.2 MHz and hearing them "flicker" out of existence as it got dark and we re-did the "next to the panel" measurements - this time the spectrum was devoid of the 200 kHz-spaced carriers (they were no longer audible on the amateur receiver, either) and the 0-30 MHz plots were 10s of dB lower than in the daylight. 

Important:  The 2 MHz sweeps in Figures 3-7 use a resolution bandwidth of 10 kHz which is almost exactly 4 times wider than the typical SSB bandwidth of an amateur receiver of about 2.5 kHz making their apparent level above the background noise appear lower than it is actually is.

What this means is the coherent signals - such as the 200 kHz carriers - appear to be another 6 dB farther above the noise floor in an SSB bandwidth than what the analyzer plots show.

Plots from a distance

Having captured some "close-in" plots, we now had an idea as to what the signals emitted by the PV system looked like.

A few days after we made the above plots we were in a virtual meeting with the manufacturer of the PV system (SolarEdge) from the ham's shack.  Having reconfigured the feed to his main radio, we could quickly switch the feedline from the antenna feeding the radio and the spectrum analyzer.

At this time we also learned that there was a second SolarEdge system south of this amateur's QTH - about 150 feet (45 meters) away across the cul de sac - and that the neighboring system and the one across the street would but remotely shut down, in that order, to determine how much QRM was emanating from each.

While we captured 0-30 MHz plots of each stage of system shutdown, for the purposes of this article we'll show just the "narrow" plots in 2 MHz sweeps as depicted in Figure 3 as the presence of the 200 kHz signal are generally representative of the presence of the broadband noise as well and these signals were easily identifiable and now known to be indicators of QRM from this type of PV system.

First, here's the plot from the amateur's 40 meter inverted Vee antenna with both systems on:

Figure 4:  6-8 MHz plot from the 40 meter antenna showing the 200 kHz peaks - and a bit of broadband noise as well.

 

The next plot shows the effects when the neighboring system was turned off, but the one across the street still on:

Figure 5:  The neighboring system off - but the one across the street still on.

As can be seen, the broadband noise floor around the 40 meter band (approximately one horizontal division below and above the marker) has dropped visibly - around 3-4 dB - and the amplitude of the carrier at 7.2 MHz has dropped about 6 dB - and the 200 kHz signals have disappeared almost entirely below about 6.5 MHz.  The system across the street was then shut off and the only remaining signals were those that happened to be on the 40 meter band.  (No trace is available for this configuration, unfortunately.)

As the 40 meter inverted Vee is oriented to favor east-west signals it was not necessarily the best candidate to test the effects of the PV system across the street, so we switched to a 20 meter antenna which was responsive in that direction and this trace shows the plot between 13 and 15 MHz:

Figure 6:  This plot of the 20 meter band and surrounding frequencies shows only propagated signals, with no sign of PV system QRM.

As both systems were off, the trace was quite clear - only showing signals that actually were on or near the 20 meter band, propagated from elsewhere in the world.  The folks at SolarEdge then turned on the system across the street with the following result:

Figure 7:  Same as Figure 6, but with only the PV system across the street activated.

The effect is clear:  In the vicinity of the 20 meter band, the appearance of rather strong signals every 200 kHz is apparent - and there is an obvious 2-4 dB increase in the noise floor indicating that this system, too, is causing harmful interference.

Readings on the radio:

It would seem that the folks at SolarEdge had worked with more than one amateur radio operator on similar issues and I was pleasantly surprised when they asked for some "S-Meter" reading comparisons with the neighbor's system on and off.  Using a calibrated signal generator, I'd already determined the signal level (in dBm) that correlated with the S-meter readings for the Icom radio - and here are the results for 40 meters:

Both systems off:

S1 (<= -84 dBm) - no carrier groups every 200 kHz.

Neighbor system on:

S4 (-78 dBm) - white noise between 200 kHz carriers.

S9 (-67 dBm) - carriers at 7.2 MHz.

This shows that at 40 meters, the degradation to noise alone was on the order of 6 dB (most Japanese radios are calibrated for 3dB per S-unit) and that the cluster of carriers on 200 kHz intervals was far more destructive, rising a bit short of 20dB out of the noise floor.

As our time with the SolarEdge folks in the virtual meeting was limited, we were not able to do similar "S-meter" tests on 20 meters, but we can use the 40 meter results along with the relative strength of the 200 kHz-spacing carriers  and correlate them with the 40 and 20 meter spectrum analyzer traces and determine that the severity of QRM from the PV system on 20 meters on the receiver would have been roughly comparable to that on 40.

Analysis of these readings and implications:

As mentioned earlier, there are two types of interfering signals produced by these SolarEdge PV systems:

• Moderate to strong clusters of carriers every "even" 200 kHz.  These are very obvious, easy to identify, and quite strong compared to the noise with a few weaker signals in-between that were also clearly audible.
  

• Background "white" noise amplitude-modulated at the mains frequency.  This is also present, but it borders on insidious as the average amateur may not be able to quantify its existence - let alone its effects - as its effects may be obscured if one only listens for it using SSB modes.

"Will my radio's DSP help?"

The quick answer is "No".

While you might think that modern receivers' ability to "notch out" tones might help alleviate the effects of the signals every 200 kHz, you would be wrong.

It appears that each, individual optimizer module (there is one for every solar panel) produces these signals - and being based on individual oscillators, their frequencies will be slightly different from each other meaning that instead of needing to notch just one tone, your DSP would have to notch out dozens emanating from a single PV system - and it just cannot do that!  What's worse, these carriers are also modulated by the low-rate data used to communicate to/from each, individual module which broadens their spectrum as well.

As for the "white" noise, it is unlikely that noise reduction would help much, either:  The source of this appears to be an artifact of the actual voltage converters themselves and as it is random, it is as difficult to reduce in its effects as the normal background noise of the bands.

As each optimizer module contains is own switch-mode power converter to maximize panel efficiency, they, too - like any switch-mode supply - will produce harmonic energy.  It would appear that SolarEdge uses switch-mode controllers that employ "spread spectrum" clocking so that instead of having a myriad of harmonics and birdies all throughout the RF spectrum, that energy is "smeared" all over the place making it somewhat less obtrusive.

The use of spread-spectrum clocking is very widely used these days for the reasons noted above - and for the fact that it also enables the exploitation of a quirk when a device is subjected to testing for regulatory (FCC) compliance:  Aspects of that testing specify the maximum amount of signal energy that may be present in a given bandwidth - but by "spreading" it over a much wider bandwidth, that same amount of energy would be diluted and make the readings obtained during the testing appear lower.  This is perfectly legal and commonly done - but this technique does nothing to reduce the total amount of energy radiated - only filtering can do that!

It is apparent that in this particular case, both the neighboring system and the one across the street contribute a magnitude of interference that would be considered to be "harmful" in that it is perfectly capable of submerging weak-to-moderate signals into locally-generated noise - and if such signals happened to land near a 200 kHz harmonic rather than the elevated noise floor in between the effects are >10dB more destructive.

It is also apparent that the radiated noise extends - at the very least - from the 40 meter to 20 meter bands (7-14 MHz) but the 30 MHz plots imply a significant amount of RF energy above and below this:  The limited time permitted a semi-detailed analysis of only the interference around the 40 and 20 meter bands.

After the meeting:

At the conclusion of these tests, the analyzer readings that took were forwarded to the folks at SolarEdge for their analysis - and it is still too soon to know of any conclusions that would indicate what sort of actions that they might take.  We were, however, heartened to know that they seemed to understand and were sympathetic to the plights of amateurs affected by neighboring systems that might be adversely affect amateur radio operation.

The folks at SolarEdge themselves offered the best hope of resolution:  They noted that they have a special version of PV hardware (e.g. optimizers) that has additional filtering that could be retrofitted into an existing system to reduce the potential for interference.  As this retrofit would be done on their "dime" - and it would be rather expensive - they understandably want to be sure that they have identified only systems that are of their manufacture that are causing interference.

Is a system near you?  You can listen for yourself!

Somewhat ominously, I have since tuned to 14.2 and 14.4 MHz (and at 7.2 MHz) on my mobile HF station while driving around residential and interstate roads in my local area (Salt Lake City, Utah) during my normal commute/business:  I can, in many places, hear the characteristic "roar" of narrow carriers every 200 kHz - likely from SolarEdge PV - systems, and these carriers seem to disappear during the hours of darkness.

I have heard this characteristic signal even in locations that appear to be several city blocks from any structure that might be equipped with a PV system.  They may also be heard on other bands - including 40 meters - but the signals emitted on the higher bands (e.g. 20 meters) seem to be emitted with greater efficiency.

It would seem that these 200 kHz-spaced groups of carriers really get out!

"I have interference from a PV system - what should I do?"

At this point I will not reiterate in detail remediation methods that might be undertaken by a radio amateur affected by this type of PV system:  The June, 2016 QST article (link) discusses attempted mitigation using ferrite devices in detail. (Note:  This article also refers to experiences with a SolarEdge system - but the spectra of the system described there is different from what I found on the systems described here likely due to it being a now-older system.)

 I will only mention in passing that there's the possibility that a degree of mitigation may be possible with the use of "noise cancelling" antennas of the sort offered by Timewave, MFJ and others - but their utility is also somewhat limited owing to practical concerns:   Such techniques work best on distant "point sources" of interference rather than very nearby, spread-out (in terms of subtended agle) radiators in the near field.

If you have interference from a PV system, it is up to YOU to do your due diligence to determine that it is, in fact, a PV system that is causing the issues and NOT other devices in your house or those of your neighbors that is causing the problem.  If you own a PV system - or have one installed on your house - that you suspect is causing a problem, making detailed measurements with it on and off on various frequencies would be a suggested first step.

As this article relates only to the SolarEdge PV system that I investigated, I cannot possibly offer advice to another brand of system that uses other brands of equipment in regards to interference potential - but if you suspect that you or your neighbor(s) have this brand of PV system that is causing interference, I would suggest the following checks during daylight and hours of darkness as appropriate:

• Are there signals every 200 kHz?  Common frequencies where this would be observed include 3.6, 3.8, 4.0, 7.0, 7.2, 14.0, and 14.2 MHz.  This is definitely one of the hallmarks of a SolarEdge system of the same/similar model - but it similar artifacts may be produced by others.
  

• Does the "hiss" that is elevating your noise floor have an obvious "hum" to it when you switch to AM?  You can't easily hear this when you are in SSB mode.  Listen for this on frequencies in the vicinity of 60, 40, 30 and 20 meters on a frequency devoid of other signals.
  

• Does the "hummy hiss" greatly reduce when it gets very cloudy?  The "hummy hiss" - which appears to be a property of the voltage converters - seems to become more intense with increased output from the PV system.
  

• Do the 200 kHz signals and the "hummy hiss" go away after sunset and return only after sunrise?  Not unexpectedly, this is hallmark of many PV systems' noise generation.
• Be aware that some models/brands (although not the one discussed in this article) can produce RF interference if either solar illumination OR mains voltage is present and that it takes the removal of BOTH to silence them (e.g. turning of the mains breaker feeding the system at night.)

If you believe that you are being affected by a PV system, it is up to YOU to be prepared to take all appropriate measures to document the interference, do your own testing, and make repeated observations prior to reporting them to the manufacturer, a regulatory agency, club or national organization.  A few things to consider:

• Treat this as if you were causing interference to someone else.  Just as if a neighbor complained that you were causing problems to their equipment, it is incumbent on YOU to determine if the problem is on your end.  There are likely many, many devices in your house that can cause similar types of interference so be sure that you have ruled those out - and DO NOT forget that you may have devices running on UPSs or battery back-up that may still make noise even if you shut off your power.  (Many UPSs are known to be noisy in their own right!)  In other words, be certain that your house is clean before involving them as this will not only make determining the magnitude/nature of interference from a PV system easier, but it shows good will and competence on your part.
  

• Document the issue over the period of days, weeks or even months.  Many sources of interference come and go - but if it's a PV system, it will be there day in and day out.  Noting over time the consistency of the noise may give you a clue if it's some other type of device - and if it, in fact, related to a PV system: GOOD documentation will only help your case.
  

• Once you have ruled out everything else, go ahead and contact the manufacturer - but be nice!  If you are confident that your own house is in order (e.g. you have ruled out other devices) then contact the manufacturer.
• If you have been following the above steps, you will already have some documentation which makes your specific case more solid.
• The manufacturer may schedule an online meeting to discuss the issue and run tests.  Be sure that you have the ability to use Zoom or Google Groups - or find someone who does.
• If the manufacturer runs tests, they will likely turn on/off suspected systems so YOU should be ready to document changes in noise floor - and of the signals every 200 kHz (in the case of a SolarEdge system of the type investigated here).  If you have already been taking notes/documenting, you should be already familiar with your local signal environment and be able to expedite the running of these tests - and have a basis of comparison as well.
• If the manufacturer decides that they wish to help remedy your situation, remember that they may be doing it at their own expense:  It is incumbent on YOU to be cooperative, competent, courteous, accurate and honest when you are dealing with them and their requests.
• If you feel the need to do so, you may wish to enlist the help of one or more friends to help you with these tasks that may be more experienced - and having a second or even third pair of eyes on the problem is always a good idea.  If you are not comfortable doing so, I would suggest have someone else - familiar with your problem - who can talk "nerd" be your spokesperson when dealing with the manufacturer!
• You should be clear to the manufacturer to define "interference" differently from "harmful interference".  If you can just hear weak birdies that don't really cause any issues, this could be considered just plain, old "interference" and you may not get as much sympathy or action as you like.  "Harmful interference" is that which - when present - obliterates even moderately strong signals that would otherwise be quite usable and thus, they should be taken more seriously.
  

While this article is rather specific to the SolarEdge PV system as described, this is likely be applicable to other manufacturers and models in more general ways.

Good luck!

* * * * *

P.S.  Overall, I was pleased with the knowledge and responsiveness of the SolarEdge folks with respect to interference to amateur radio stations.  After they have had time to digest the information supplied and executed their plan of action I hope to do a follow-up to ascertain the results of their mitigation efforts.

Myself and several other local amateur radio friends have PV (solar) at our own QTHs and experience ZERO interference.  As we had chosen to take an active part in our PV system design, we had installed SunnyBoy series-string systems which are known (and proven!) to have zero interference potential on any LF, MF or HF amateur band as described in the link(s) below.  Unfortunately, some installers will not entertain the use of this type of system if it is not in the suite of products that they offer.

Other local amateurs that I know have microinverter-based PV systems using Enphase IQ modules and have reported minimal or no interference.  As I have not (yet) had the opportunity to carefully analyze the spectral signature of this product, I can only go by their assertion that their own system has not caused them obvious problems.  I hope to do a careful analysis of a modern Enphase system and if so, I'll report the findings on this blog.

Follow-up - post mitigation:

In the late fall of 2024 the folks at SolarEdge - at their expense - swapped out the "standard" optimizers with special versions that are "RF Quiet".  This amateur reported that while the QRM has been reduced, it is still quite obvious:  I have not returned to his QTH to do any comparative measurements.

While it's good that SolarEdge has done work to mitigate the interference, it's unfortunate that the "fix" (swapping out the optimizers and possibly other things) didn't result in extinction of the interference.  The fact that SolarEdge has a "normal" and "RF Quiet" version of this product is also a bit disturbing in it's own right! 

Please post in your comments your experiences with PV systems - but please do so in the context of having fully read this article and at least perused the articles linked below.

  * * * * *

Other articles at this blog on related topics:

• It *IS* possible to have an RF-quiet home PV (solar) electric system - link
  
• The Solar Saga Part 1: Avoiding Interference - (Why I did not choose Microinverters) - link
  
• The Solar Saga Part 2:  Getting the system online - link
• Does the Tesla Powerwall 2 produce Radio Frequency Interference? - link
• Reducing RFI from the Tesla Powerwall 2 - link 
• Tesla Powerwall susceptibility to RF Transmissions - and how to deal with it - link 
  

Other articles related to this topic:

• Solaredge Found in Breach of Swedish Electromagnetic Rules

 

Stolen from ka7oei.blogspot.com

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Odd differences between two (nearly) identical PV systems

I've had my 18-panel (two groups of 9) PV (solar) electric system in service for about a year and recently I decided to expand it a bit after realizing that I could do so, myself, for roughly $1/watt, after tax incentives.  An so it was done, with a bit of help from a friend of mine who is better at bending conduit than I:  Another inverter and 18 more solar panels were set on the roof - all done using materials and techniques equal to or better than that which was originally done in terms of both quality and safety.

Adding to the old system:

The older inverter, a SunnyBoy SB 5000-TL, is rated for a nominal 5kW and with its 18 panels, 9 of each located on opposite faces of my east/west facing roof (the ridge line precisely oriented to true north-south) would, in real life, produce more than 3900 watts for only an hour or so around "local noon" on late spring/early fall summer days that were both exquisitely clear and very cool (e.g. below 70F, 21C).  I decided that the new inverter need not be a 5kW unit so I chose the newer - and significantly less expensive SunnyBoy SB3.8 - an inverter nominally rated at 3.8kW.  The rated efficiencies of the two inverters were pretty much identical - both in the 97% range.

Figure 1:
The installed 3.8 kW inverter in operation with the 2kW
"SPS" (Secure Power System) island power outlet shown below.
Click on the image for a larger version.

One reason for choosing this lower-power inverter was to stay within the bounds of the rating of my main house distribution panel.  My older inverter, being rated for 5kW was (theoretically) capable of putting 22-25 amps onto the panel's bus, so a 30 amp breaker was used on that branch circuit while the new inverter, capable of about 16 amps needed only a 20 amp breaker.  This combined, theoretical maximum of 50 amps (breaker current ratings, not actual, real-world current from the inverters and their panels!) was within the "120% rule" of my 125 amp distribution panel with its 100 amp main breaker:  120% of 125 amps is 150 amps, so my ability to (theoretically) pull 100 amps from the utility and the combined capacity of the two inverters (again, theoretically - not real-world) being 50 amps was within this rating.

Comment:  The highest total power that power that I have seen from my system has been about 8000 watts - 3900 watts from the SB3.8 and just over 4100 watts from the SB 5000 for a maximum of about 36 amps at 220 volts (abnormally low line voltage!) or about 33 amps total with a more typical 240 volt feed-in - well under the "50 amp" maximum.

For the new panels I installed eighteen 295 watt Solarworld units - a slight upgrade over the older 285 watt Suniva modules already in place. In my calculations I determined that even with the new panels having approximately 3.5% more rated output (e.g. a peak of 5310 watts versus 5130 watts, assuming ideal temperature and illumination - the latter being impossible with the roof angles) that the new inverter would "clip" (e.g. it would hit its maximum output power while the panels were capable of even more power) only a few 10s of days per year - and this would occur for only an hour or so at most on each occasion.  Since the ostensibly "oversized" panel array would be producing commensurately more power at times other than peak as well, I was not concerned about this occasional "clipping".

What was expected:

The two sets of panels, old and new, are located on the same roof with the old array being higher, nearer the ridge line and the new being just below.  In my situation I get a bit of shading in the morning on the east side, and a slight amount in the very late afternoon/evening in mid summer on west side and the geometry of the trees that do this cause the shading of both the new and old systems to be almost identical.

With this in mind, I would have expected the two systems to behave nearly identically.

But they don't!

Differences in produced power:

Having the ability to obtain graphs of each system over the course of a day I was surprised when the production of the two, while similar, showed some interesting differences as the chart below shows. 

Figure 2:
The two systems, with nearly identical PV arrays.  The production of the older SB5000 inverter with the eighteen 285 watt panels is represented by the blue line while the newer SB3.8 inverter with eighteen 295 watt panels is represented by the red line:  Each system has nine east-facing panels and nine west-facing panels.  The dips in the graph are due to loss of solar irradiance due to clouds.  Because the data for this graph is collected every 15 minutes, some of the fine detail is lost so the "dip" in production at about 1:45PM was probably deeper than shown.
The total production of the SB3.8 system (red line) for the day was 27.3kWh while that of the SB5000TL system (blue line) was 25.4kWh - a difference of about 7% overall.
Click on the image for a larger version.

In this graph the blue line is the older SB5000TL inverter and the red line is the newer SB3.8 inverter.  Ideally, one would expect that that the newer inverter, with its 295 watt panels, would be just a few percent higher than the older inverter with its 285 watt panels, but the difference, particularly during the peak hours, is closer to 10%, particularly during the peak times when there is no shading at all.

What might be the cause of this difference?

Figure 3:
 The two parallel east-facing arrays, the older one being closer to
the (north-south) peak of the roof.
Click on the image for a larger version.

Several possible explanations come to mind:

1. The new panels are producing significantly more than their official ratings.  A few percent would seem likely, but 10%?
2. The older panels have degraded more than expected in the year that they have been in service.
3. The two manufacturers rate their panels differently.
4. There may be thermal differences.  The "new" panels are lower on the roof and it is possible that the air being pulled in from the bottom by convection is cooler when it passes by the new panels, being warmer by the time it gets to the "old" panels.  If we take at face value that 3.5% of the 10% difference is due to the rating - leaving 6.5% difference unaccounted, this would need only about a 16C (39F) average panel temperature difference, but the temperature differences do not appear to be that large!
5. The new panels don't heat up in the sun as much as the old.  The new panels, in the interstitial gap between individual cells and around the edges are white while the old panels are completely black, possibly reducing the amount of heating.  Again, there doesn't seem to be a 16C (39F) difference.
6. The new inverter is better at optimizing the power from the panels than the old one.

It's a bit difficult to make absolute measurements, but in the case of #2, the possibility of the "old" panels degrading, I think that I can rule that out.  In comparing the peak production days for 2016 and 2017, both of which occurred in early May (a result of the combination of reasonably long days and cool temperatures) the best peak was about the same - approximately 28.25kWh on the "old" system even after I'd installed the "new" panels on the east side.

I suspect that it is a combination of several of the above factors, probably excluding #2, but I have no real way of knowing the amount of contribution of each.  What is surprising to me is that I have yet to see any obvious clipping on the new system on the production graphs even though I have "caught" it pegged at about 3920 watts on several occasions during local noon, so it seems that my calculation of "several dozen of hours" per year where this might happen is about right.

I'll continue to monitor the absolute and relative performance of the two sets of panels to see how they track over time.

* * *

Update - June, 2018:

The differences noted above still persist.  In comparing the overall power production results between this year and last, there are no obvious changes in the two systems.


[End]

This page stolen from "ka7oei.blogspot.com"

Teasing out the differences between the "AC" and "DC" versions of the Tesla PowerWall 2

Being naturally interested in such things, I've been following the announcements and information about the Tesla PowerWall 2 - the follow-on product of the (rarely seen - in the U.S., at least) "original" PowerWall.

Somewhat interestingly/frustratingly, clear, concise (and even vaguely) technical information on either version of the PowerWall 2 (yes, there are two versions - the "DC" and "AC") has been a bit difficult to find, so in my research, what have I found?

Comment:  It would appear that the "DC" version of the PowerWall 2 has been discontinued - or, at the very least, it not routinely offered.

This page or its contents are not intended to promote any of the products mentioned nor should it be considered to be an authoritative source. It is simply a statement of opinion, conjecture and curiosity based on the information publicly available at the time of the original posting. It is certain that as time goes on that information referenced on this page may be officially verified, become commonplace, or proven to be completely wrong. Such is the nature of life!

The "DC" PowerWall 2:

• Data sheets (two whole pages, each - almost!) for both the DC and AC versions of the PowerWall may be found here at this link - link.

Unless you have a "hybrid" solar inverter, this one is NOT for you - and if you had such an inverter, you'd likely already know it.  A "hybrid" inverter is one that is specifically designed to pass some of the energy from the PV array (solar panels) into storage, such as a battery and used that stored energy later.

Unlike its "AC" counterpart (more on this later) this version of the PowerWall 2 does NOT appear to have an AC (mains) connection of any type - let alone an inverter (neither are mentioned in the brochure) - but rather it is an energy back-up for the solar panels on the DC input(s) of the hybrid inverter.   "Excess" power from the panels may used to charge the battery and this stored energy could be used to feed the inverter when the load (e.g. house) exceeds that available from the panels - when it is cloudy, if there is a period in which the load exceeds the output of the PV array for a period of time or there is no sun at all (e.g. night).

Whether or not this version of the PowerWall can actually be (indirectly) charged via the AC mains (e.g.  via a hybrid inverter capable of working "backwards" to produce AC from the mains) would appear to depend entirely on the capability and configuration of the hybrid inverter and the system overall.

But, you might ask,why would you ever want to charge the battery from the utility rather than from solar?  You might want to do this if there were variable tariffs in your area - say, $0.30/kWh during the peak hours in the day, but only $0.15kWh at night - in which case it would make sense supplant the "expensive" power during the day with "cheap" power bought at night to charge it up:  Although there would be, perhaps, a 10% "round trip loss" in doing this, it would still save money overall and help "even out" the loading that a utility might see during peak hours.

Whether or not this system would be helpful in a power outage is also dependent on the nature of the inverter to which it is connected:  Most grid-tie solar converters become useless when the mains power disappears (e.g. cannot produce any power for the consumer - more on this later) - and this applies to both "series string" (e.g. a large inverter fed by high-voltage DC from a series of panels) and the "microinverter" (small inverters at each of the panels) topologies.  Inverters configured for "island" operation (e.g. "free running" in the absence of a live power grid) or ones that can safely switch between "grid tie" and "island" modes would seem to be appropriate if you use the DC PowerWall and you want to keep your house "powered up" when there is a grid failure.

In other words, if you have a typical PV system that involves grid-tie inverters (series string or microinverter) and you have no "islanding" capability at present, the "DC" Power Wall is not for you!

The "AC" PowerWall 2:

• Data sheets (two whole pages, each - almost!) for both the DC and AC versions of the PowerWall may be found here - LINK.

While the "AC" version seems to have the same battery storage capacity as the "DC" version (e.g. approx. 13.5kWh) it also has an integrated inverter and charger that interfaces with the AC mains that is apparently capable of supporting any standard voltage from 100 to 277 volts, 50 or 60 Hz, split or single phase.  This inverter, rated for approximately 7kW peak and 5-ish kW continuous, is sufficient to run many households.  Multiple units may be "stacked" (e.g. connected in parallel-type configuration - up to nine of them, according to the data sheet linked above) for additional storage and capacity.

Unlike the "DC" version, all of the power inflow/outflow is via the AC power feed, which is to say, it will both output AC power via its inverter and charge its battery via that same connection.  What this means is that it need not (and cannot, really) be directly connect to the PV (photovoltaic) system.  What seems clear is that this version has some means of monitoring the net flow in to and out of the house and to/from the utility which means that the PowerWall could balance this out by "knowing" how much power it could use to charge its battery, or needed to output.

(The basic diagram of Figure 1, below, shows how such a system might be connected.  This diagram does not specifically represent a PowerWall, but rather how any battery-based inverter/charger system might be used to supply back-up power to a home in the past and future.)

Because its power would be connected "indirectly" via AC power connections to the PV system it should (in theory) work with either a series-string or microinverter-type system - or, maybe even if you have no solar at all if you simply want to charge it during times of lower tariffs and pull the charge back out again during high tariffs.

(The Tesla brochure simply says "Support for wide range of usage scenarios" under the heading "Operating Modes" - which could be interpreted many ways, but at the time of the original posting of this article I have not actually seen an "official" suggestion of a use without any sort of solar power.)

What might such a system look like - schematically, at least?

How might this version of the PowerWall operate?  First, let's take a look at a diagram of how any sort of battery/inverter/charger like this might be configured for a house.

Figure 1:
Diagram of a generic battery-based "whole house" backup system based on obvious requirements.  This is a very basic diagram, showing most of the needed components that would be required to interface a battery-based inverter/charger with a typical house's electrical system and a PV (PhotoVoltaic/solar) charging system.
For those not familiar with North American power systems, typical residences are fed with 240 volt, center-tapped service from the utility's step-down transformer with this center-tap grounded at the service entrance.  This allows most devices to operate at 120 volts while those that consume large amounts of power (ranges, electric water heaters, electric dryers, air conditioners, etc.) are connected to 240 volt circuit, which may or may not need the "neutral" lead at all.  In most other parts of the world there would be only "L1" and the "Neutral" operating at about 240 volts.
Click on the image for a larger version.

Referring to Figure 1, above:

Shown to the right of center is a switch that opens when the utility's power grid goes offline, isolating the house and the inverter/charger from the power grid and included in that is a voltage monitor (consisting of potential transducers, or "PTs") that can detect when the mains voltage has returned and stabilized and it is "safe" to reconnect to the grid.  The battery-based inverter/charger is connected across the house's mains so that it can both pull current from it to charge its battery as well as push power into the house in a back-up situation.

The "Net current monitoring" current transducers ("CTs") might be used to allow the inverter/charger to "zero out" the total current (and, thus power) coming in from and going out to the power grid (under normal situations) such as when its battery is being charged and extra power is being produced by the PV system, but also to control the charge rate just so that only that "extra" power from the PV system is being used to assure, as much as possible, a net-zero flow to/from the utility.  The "House Current monitoring" is used to determine how much current is being used by the entire house while the "PV current monitoring" is used to determine the contribution of the PV system.

Comment:

The "PV current monitoring" point is probably superfluous:  All the PowerWall need know is how much power is going in to or out of the utility (power grid) and how much power is going in to or out of the house's main panel, which also includes any solar generation.  The third factor - how much power the PowerWall is using/producing is going to be available from the PowerWalls' own built-in monitoring.

By knowing these things it is possible to determine how much excess/deficit their may be in terms of the production of the PV system with respect to actual usage by the household.  Not shown is the current monitoring that would, no doubt, be included in the inverter/charger itself.  Some of the shown current monitoring points may be redundant as this information could be determined in other ways, but are included for clarity.

Finally, a local network (data) connection is shown for both the inverter/charger and the PV system so that there is a possibility that they may communicate with each other, perhaps for control purposes, as well as communicate via the Internet so that statistics may be monitored and recorded and to allow firmware updates to be issued.

How it might operate in practice:

As can be seen in Figure 1 and determined from the explanation, we can see that the PV is connected to the input/output of the inverter/charger (which could be a PowerWall - or any other similar system) via the house wiring which means that there is a path to the PowerWall to charge its battery, and the same path out of it when it needs to supply power, along with means of monitoring power flow.

With a system akin to that depicted in Figure 1, consider these possible scenarios:

1. Excess power is being produced by the PV system and put back into the grid and the PowerWall's battery is fully-charged.   Because the battery is fully-charged there is nowhere to put this extra power so it goes back into the grid, tracked by the utility's "Net Meter" in the same way that it would be without a PowerWall.
2. Excess power is being produced by the PV system and the PowerWall's battery is not fully charged.  The PowerWall will pull the amount of "excess" power that the PV system would normally be putting into the grid and charge its own battery at that same rate resulting in a net-zero amount of power being put into the grid.
3. More power is being consumed by the user's household than is being produced by the solar array.  Depending on the state-of-charge and configuration of the PowerWall it may produce enough power to make up for the difference between what the PV system is producing and the user's needs.  At night this could (in theory) be 100% of the usage if the system were so-configured.
4. Tariff leveling.  It would be theoretically possible to configure it so that whether or not solar was present and the utility charged a higher daytime than nighttime power rate, one could charge overnight from the mains and put out power during the day to reduce the power costs overall and to help "level" the utility's load.

What about a power outage?

All of the above scenarios are to be expected - and they are more-or-less standard offerings for many of the battery-based products of this type - but what if the AC mains go down?  For the rest of this discussion we will ignore the "DC" version of the PowerWall as its capability would rely on the configuration of the user's hybrid inverter and its capabilities/configuration when it comes to supplying backup, "islanded" AC power although the combination of a DC power wall and the appropriate inverter could be functionally identical to an AC Power Wall.

As mentioned before, with a typical PV system - either "series string" (one large inverter) or distributed (e.g. "microinverter") - if the power grid goes offline the PV system becomes useless:  A PV system requires the power grid to be present to both synchronize itself and present an infinite "sink" into which it can always "push" all of the "extra" power power that it is producing.  Were such units to not shut down, dangerous voltages could be "back-fed" into the power grid and be a hazard to anyone who might be trying to repair it.  It is for this reason that all grid-tie inverters are, by law, required to go offline and/or disconnect themselves completely from the power grid during a mains power outage.

The "AC" version of the Tesla PowerWall's system includes a switch that automatically isolates the house from the utility's power grid when there is a power failure.  Once this switch has isolated the house from the power grid the inverter built into the PowerWall can supply power to the house - at least as long as its battery lasts.

What about charging the battery during a power outage?

Here is where it seems to get a bit tricky.

If all grid-tie inverter systems go offline when the power grid fails, is it possible to use it to assist, or even charge the PowerWall during a grid failure?  In other words, can you use power from your PV system to recharge the PowerWall's battery or, at the very least, supply at least some of the power to extend its battery run-time?

In corresponding with a company representative - and corroborated by data openly published by Telsa (see the FAQ linked near the bottom of this posting) - the answer would appear to be "yes" - but exactly how this works is not very clear.

Based on rather vague information and knowing the behavior of the components involved it would seem to need to work this way:

• The power (utility) grid goes down.
• The user's PV system goes offline with the failure of the grid.
• The PowerWall's switch opens, isolating the house completely from the grid - aside from the ability to monitor when the power grid comes back up.
• The inverter in the PowerWall now takes the load of the (now isolated) house, producing AC power.

Were this all that happened, the house would again go dark once the battery in the PowerWall or similar "back-up power system" was depleted, but there seems to be more to it than this when a PV system is involved, as in:

• When the back-up power system's inverter goes online, the PV system again sees what looks like the power grid and comes back online.
• As it does, the back-up power system monitors the total power consumption and usage and any excess power being produced by the PV system is used to charge its battery.
• If the PV system is producing less power than is being used, the back-up power system will supply the difference:  Its battery will still be discharged, but at a lower rate.  The house will still go dark when the battery is fully discharged.

Comment: 

What if you run the Power Wall down to the point where it goes offline and then the sun comes out the next day:  Is it possible to "bootstrap" the system to cause the PV to go online and start charging the battery, or are you "stuck" in an "offline" state where you can't produce PV to charge the battery because there is no AC power, but you can't produce AC power to charge the battery?

It is entirely possible that the DC version of the Power Wall may be "immune" to this "catch-22" situation by allowing some "reserve" capacity to restart once PV power is again available for charging - but how would it know that?

But now it gets even trickier and a bit more vague.

What if there is extra power being produced by the PV system?

Grid tie PV systems expect the power grid to be an infinite sink of power - but what if, during a power failure, when your backup up system is standing in as the power grid, your PV system is producing 5kW of solar energy and your house/inverter is using only 2kW:  Where does the extra 3kW of production go if it cannot be infinitely sinked into the utility grid, and how does one keep the PV system from "tripping out" and going off line?

To illustrate the problem, let us bring up a related scenario where we have a generator instead of some sort of battery-based back-up power system system.

There is a very good reason why owners of grid-tie systems are warned against using it to "assist" a backup generator and using that generator as a substitute for the power grid.  What can happen is this:

• The AC power goes out and the transfer switch connects the house to the generator.
• The generator comes online and produces AC power.
• If the AC power from the generator is stable enough (not all generators produce adequately stable power) the PV system will come back online thinking that the power grid has come back.
• When the PV system comes back online and produces power, the generator's load decreases:  Most generator's motors will slightly speed up as the load is decreased.
• When the generator's motor speeds up, the frequency goes high.  When this happens, the PV system will see that as unstable power and will go offline.
• When the PV system goes off, the power is suddenly dumped on the generator and it is hit with the full load and slows back down.
•  The cycle repeats, with the PV system and generator "fighting" each other as the PV system continually goes on and offline.

An even worse scenario is this:

• The AC power goes out, the transfer switch connects the house to the generator.
• The generator comes online and produces power.
• The PV system comes up because it "sees" the generator as the power grid, but its producing, say, 5kW but the house is, at the moment, using 2kW.
• The PV system, because it think that it is connected to the power grid, will try to shove that extra 3kW somewhere, causing one or more of the following to happen:
• The generator to speed up as power is being "pushed" into it, its frequency will go high and trip the PV system offline, and/or:
• If the PV system tries to push more power into the system than there is a place for it to go (e.g. the case, above, where the solar is producing 3kW more than is being used) the voltage will necessarily go up.  Assuming that the generator doesn't "overspeed" and trip-out and the frequency doesn't go up and trip the PV system offline, the PV system will increase the voltage, trying to "push" the extra power into a load where there is nowhere for it to go:
• As the PV system tries to "push" its excess power into the generator, it will increase the output voltage.  At some point the PV system will trip out on overvoltage, and the same "on-off" cycle mentioned above will occur.
• It is possible that the excess power from the PV will "motor" the generator (e.g. the input power tries to "spin" the generator/motor) - an extremely bad thing to do which will probably cause it to overheat and eventually be destroyed if this goes un-checked.
• If it is an "inverter" type generator, it can't be "motored", but the excess power will probably cause the generator's inverter to get stuck in the same "trip out/restart" cycle or simply fault out in an "overload condition - or the inverter might even be damaged/destroyed.

If having extra power from a grid-tie inverter is so difficult to deal with, what could you do with extra power that the PV system might be producing?

What if we have excess power and nowhere to put it?

The question that comes to mind now is "What does the PV system do when the PowerWall's battery is fully-charged and there is no-where to put extra energy that might be being produced?"  Where we have is the situation where our PV system is producing 5kW but we are using only 2kW leaving an extra 3kW to go... where?

The answer to that question is not at all clear, but four possibilities come to mind:

1. Divert the power elsewhere.  Some people with "island" systems utilize a feature of some solar power systems that indicate when excess power is available and use it to operate a diversion switch to shunt the excess power in an attempt to do something useful like run an electric water heater, pump water or simply produce waste heat with a large resistor bank.  Such features are usually available only on "island" systems (e.g. those that are entirely self-contained and not tied to the power grid) and with large battery banks.
2. Disable the PV system temporarily.  If it is possible, simply disable the PV system for a while and drain, say, 5-10% of the power out of the back-up power system battery before turning it back on and recharging it.  This will cause the PV system to cycle on and offline, but it will do so relatively slowly and it should cause no harm.
3. Tell the PV system to shut off.  One could somehow communicate with the PV system and "tell" it to produce only the needed amount of energy.  This is a bit of a fine line to walk, but it is theoretically possible provided such a feature is available on the PV system.
4. Alter the power to cause the PV system to drop off-line.  One could, in theory, alter the conditions of the power being produced by the back-up power system inverter such that it causes the PV system to go offline and stay that way until it needs to come back online.

Analyzing the possibilities:

Let's eliminate #1 as that will not apply to a typical grid-tie system, so that leaves us with:

#2:  Disabling the PV system:

Of these three possibilities #2 would seem to be the most obvious and it could be done simply by having another switch/relay on the output of the PV system that disconnects it from the rest of the house, forcing it to go offline - but this has its complications.

For example, in my system the PV is connected into a separate sub-panel located in the garage:  If one were to disconnect this branch circuit entirely, the power in the garage would go on and off, depending on the state-of-charge of the PowerWall or other battery-based back-up power system.  Connecting a PV system to a sub-panel is not an unusual configuration as it is not uncommon to find them connected to sub-panels that feed other systems, say, the air conditioner, kitchen, etc. (e.g. wherever a suitable circuit is available) so I'm guessing that they do not do it this way - unless they do it at the point before the PV system connects to the panel.  Doing this would require a remotely-controlled switch in many situations - awkward to wire up in many situations, but not impossible.

As noted above, one would disable the PV system once the battery had fully charged but enable it again once the battery had run down a bit - say, to 90-95%.  This way, one would not be rapid-cycling the PV system and the vast majority of the back-up power system's battery storage capacity would be available.

While this, too, should work, I suspect that it is not the one that is used as the drawings in the brochures don't show any such connection - but then again, they don't show the main house disconnect that would have to be present - but it would probably work just fine if the PV system were to gracefully come back online when it was time to do so (e.g. no user intervention to "reset" anything.)

#4:  Alter the operating conditions to cause the PV system to go offline:

Then there is #4, and one interesting possibility comes to mind - and it may sound like a kludge, but it should work.

One of the parameters that could be altered would be the frequency at which the back-up power system's inverter operates (say, 2-3 Hz or so above and/or below the proper line frequency) and force the PV system offline with that variance.  Even though this minor frequency change is not likely to hurt anything (many generators' frequencies drift around much more than this with varying loads!) devices that use the power line frequency as a reference - such as clocks, and clocks within various appliances - would drift rather badly unless the frequency were "dithered" above and below the proper frequency so that its long term average was properly maintained.

I suspect that this is not a method that would be used, but it could work - at least in theory.

Edit - 20170719:

In digging around, I have determined that "dithering" the frequency is, in fact, one of several ways that is used by an battery-backed inverter to disable a PV inverter when the PV is producing more power than can be accommodated by the load and/or battery charger.  This system, called "Frequency Shift Power Control" (FSPC) by at least one manufacturer (e.g. SunnyBoy) is designed to do this very thing.

A description of this technique may be found in section 6 of the document "Use of PV Inverters in Off-Grid  and Backup Systems in North and  South America" by SMA (SunnyBoy) found at this link.

Whether or not this is a control method used by the Power Wall is not known at this time.

Edit - 20180502:

Now having had an operational PowerWall 2 system for some time, I can verify that it is, in fact the "frequency control" method that is used.  During a sunny day with the battery at about 90%, I disconnected my house from the utility and observed that the PowerWall was taking a charge from the PV system.  During this time I monitored the AC mains frequency in the house.

At 97-98% charge the mains frequency increased from 60.00Hz to 60.50 Hz over several minutes at which point my PV inverters shut down due to the frequency being out of tolerance.  As the charge level dropped back down to 95% or so, the frequency slowly returned to 60.00 Hz and eventually the PV inverters came back online.  I kept the mains power disconnected for several hours and the above cycle repeated continuously.

My SunnyBoy PV inverters will simply "drop" abruptly when the limit (60.50 Hz) is reached, but it is possible that there are inverters out there that will, at some point, more gradually throttle the power back above a certain frequency threshold.  While this scheme would be more "graceful", either method works just fine.

One side-effect of this method is that the average mains frequency is increased during those times where the PV inverter is to be disabled.  If we assume that one were running only on solar, and there were 3 peak solar production hours that this will occur 50% of the the time (e.g. about 90 minutes spent at 60.5 Hz during the day) this would mean that clocks that were synchronized to the mains frequencies (many plug-in digital clocks, most appliances) would run "fast" by about 45 seconds per day.  This difference is in mains frequency is small enough that motorized appliances will function just fine.

#3:  "Talk" to the PV system and control the amount of power that it is producing:

That leaves us with #3:  Communicate with the PV system and "tell" it (perhaps using the "ModBus" interface) to produce only enough power to "zero" out the net usage.

The problem with this method is that it would depend on the capabilities of the PV inverter system and require that they support such specific remote control functions.  While it is very possible that some do, this method would be limited to those so-equipped and compatibility across many brands/models could be difficult.

#3 and #2:  "Talk" to the PV system to turn it on and off as needed:

Included in #3 could be a variant of method #2 and that would be to send a command to the inverter via its network connection to simply shut down and come back online as needed to keep the battery between, say, 90% and 100% charge as mentioned above.

This second variant of #3 seems most likely as there as it is possible that there is some sort of set of commands capable of this that would be widely implemented across vendors and models.

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What do I think the likelihood to be?

I'm betting on the second variant of #3 where a command is sent to the PV system to tell it to turn off - at least until there is, again, somewhere to "send" excess power - but #4 is looking increasingly likely.

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Having said all of this, there is a product FAQ that was put out by Tesla that seems to confirm the basic analysis - that is, its ability to run "stand alone" in the event of a power failure and the charge be maintained if there is sufficient excess PV capacity - read that FAQ here - LINK.

I'm investigating getting a PowerWall 2 system to augment my PV generation and provide "whole house" backup.  In the process I have been researching how it works and interfaces with both the utility and my existing PV system. While I have occasionally asked questions of representatives of  Tesla, nothing that they have said is anything that could not be easily found in publicly-released information on the internet and as of the original date of this posting I haven't signed anything that could possibly keep me from talking about it. However all of its interfacing and connectivity is done, it should be interesting!

Additional information may be found on the GreenTech Media web site:  "The New Tesla Powerwall Is Actually Two Different Products" - LINK.  This article and follow-up comments seem to indicate that there were, at the time of their writing, there were only a few manufacturers of inverters, namely SolarEdge and SMA (a.k.a. SunnyBoy) with which Tesla was installing/interfacing their systems, perhaps indicating some version of #2 or #3, above.  Clearly, the comments, mostly from several months ago, are also offering various conjectures on how the system actually works.

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Finally, if you can find more specific information - say from a public document or from others' experience and analysis that can add more to this, please pass it along!


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