Tesla Gateway and Powerwall limitations with grid-tie and SolarEdge inverters

[turtle2472]

We recently had a system installed with 64 panels totaling ~23kW. They utilize SolarEdge optimizers and two SE7600H inverters. We are grid-tied with a Tesla Gateway and two Powerwalls. We see a max production of ~15.7kW with 10kW feeding the batteries when production exceeds consumption.

The company that did the install said we were limited to the 7600W inverters because “One powerwall’s maximum AC PV input is 7600w so with two Powerwalls, the most that can be installed is 15.2kw AC of PV". This seems artificial to me and we should be running 10kW inverters instead giving us a max output of 20kW.

Doesn't the Powerwall manage it's charging with a BMS of some sort? Why would limiting my solar production be needed due to battery size?

 

[zanydroid]

I think installer explained it wrong. Keep in mind they're not really engineers so they're somewhat more likely to say stuff that doesn't make sense, but is based on something real & sensible...

AC coupling often has design rules like that.

Imagine if you are off-grid with 30 kW-AC of PV and 15kW-AC of PowerWalls. Suppose you have 30 kW of loads consuming that. Suddenly a cloud shows up and drops you down to 5kW-AC PV. Your powerwalls cannot make up the difference, so your microgrid browns out.

Yes, those numbers are silly, adjust them until they make sense to you.

 

[turtle2472]

You scenario makes sense, but I'm grid-tied so the grid would fill in for the lack of production. So sure the solar wouldn't keep up but the Gateway would then route from the grid. If the grid were to be down too, then I would shed loads with my Span Panel to keep from overloading the Powerwall output.

I still don't get how I am limited on inverter size due to battery sizing. If I didn't even have inverters or solar the Powerwalls would charge and discharge regardless. This inverter limit of 7600W seems like the installer/designer is misreading specs.

 

[solar8484]

You are mixing different things. You have 2 SE7600H so that limits the max AC power output to ~15kW period. The 23kW number for your 64 panels is the max DC power from the solar panels to your SE7600H's. It's normal to have 1.5x DC-to-AC power ratio to ensure adequate production for low sunlight conditions (e.g. winter) and solar panel degradation over time. Your particular solar inverter output limitation has nothing to do AC coupling or Powerwall's.

 

[zanydroid]

I still don't get how I am limited on inverter size due to battery sizing. If I didn't even have inverters or solar the Powerwalls would charge and discharge regardless. This inverter limit of 7600W seems like the installer/designer is misreading specs.

The "charging" part of their statement is somewhat weird. Though that still relates to one of the design rules. Suppose you were consuming 15kW-PV from your appliances. Then the appliances all turn off at once. The PV stays the same. There are ways to eventually curtail it, but there will be some lag. Having a charger that can soak it up ensures that there is a load that is available to handle all of that to prevent a voltage spike and the power randomly going into an uncontrolled place. (And yes you need enough spare space in the battery for this, I assume PowerWall leaves a reserve in its control algorithm and discharges as needed to return to reserve to handle this case).

That will not happen on grid, but it can happen worst case when off grid. Your SPAN will probably help in a bunch of situations, but I don't know how much the PowerWall instructions permit SPAN to excuse deviating from the design rules. And they are required to follow the manufacturer installation instructions.

All that said, there is some incentive for them to sell you more powerwalls...

 

[wattmatters]

A PW2 has a continuous charge/discharge limit of 5 kW and peak power limit of 7.6 kW.

If you expect your PV system to be able to operate during a grid outage, and for the PW2 to form a microgrid with the PV, then it cannot risk a scenario where the PV array is generating more than the peak power the PW2 can handle.

Imagine 10 kW of PV generation (AC power out of the PV inverter), mostly being exported to the grid because the battery is full and not many loads.

Then the grid goes down.

That 10 kW of production has to go somewhere, and the only option is the PW2 battery. But it is not rated to accept that much power, even briefly.

Provided the peak power the PV system can supply is within the PW2's power capacity limit, the the PW2 can take it up then begin its process of forming the microgrid and raising the microgrid's AC frequency so as the throttle back the PV output to bring it back down the satisfy loads only and any battery charging which may be required.

Which is why any PV system which the PW2 has been set up to control during a grid outage must not have an AC output greater than 7.6 kW.

You can certainly have a much larger PV system than a 7.6 kW grid-tied inverter, however you cannot set it up so that the PW2 takes control of the PV system and keeps it operating during a grid outage. In this case the grid-tied PV system would immediately turn off and not comes back on until the grid comes back online. You will still be operating on backup power from the PW2, however it will not be getting any charge from the PV system.

And likewise, normally the "essential" circuits the PW2 / Gateway is set up to supply during an outage are limited to prevent the system shutting down. Since the PW2 is limited to 5 kW continuous / 7.6 kW peak, then any loads it supplies should the grid go down should not exceed 5 kW. If they do, then the breaker will trip and you'll have to reset the system.

 

[zanydroid]

Are you allowed to install PW2 with microinverters programmed to have an AC export limit when you switch off grid? EG set 7.6kW limit on a deployment of Hoymiles microinverters. Or is the 30 second response delay / wireless communications not good enough?

 

[turtle2472]

A PW2 has a continuous charge/discharge limit of 5 kW and peak power limit of 7.6 kW.

If you expect your PV system to be able to operate during a grid outage, and for the PW2 to form a microgrid with the PV, then it cannot risk a scenario where the PV array is generating more than the peak power the PW2 can handle.

Imagine 10 kW of PV generation (AC power out of the PV inverter), mostly being exported to the grid because the battery is full and not many loads.

Then the grid goes down.

That 10 kW of production has to go somewhere, and the only option is the PW2 battery. But it is not rated to accept that much power, even briefly.

Provided the peak power the PV system can supply is within the PW2's power capacity limit, the the PW2 can take it up then begin its process of forming the microgrid and raising the microgrid's AC frequency so as the throttle back the PV output to bring it back down the satisfy loads only and any battery charging which may be required.

Which is why any PV system which the PW2 has been set up to control during a grid outage must not have an AC output greater than 7.6 kW.

You can certainly have a much larger PV system than a 7.6 kW grid-tied inverter, however you cannot set it up so that the PW2 takes control of the PV system and keeps it operating during a grid outage. In this case the grid-tied PV system would immediately turn off and not comes back on until the grid comes back online. You will still be operating on backup power from the PW2, however it will not be getting any charge from the PV system.

And likewise, normally the "essential" circuits the PW2 / Gateway is set up to supply during an outage are limited to prevent the system shutting down. Since the PW2 is limited to 5 kW continuous / 7.6 kW peak, then any loads it supplies should the grid go down should not exceed 5 kW. If they do, then the breaker will trip and you'll have to reset the system.

Thank you soooo much for this explanation. It really helped me understand the issue and why the limitation is present. I think I actually get it now.

With our two PW2s and Span Panel we are able to double to 10kW continuous from the batteries and the Span sheds the loads immediately on grid dropping to prevent the overcurrent out from the batteries. So in my build everything is under the Span and it sheds the loads vs having a dedicated "essential circuits" panel or similar.

The other thing I'm figuring out is the stark difference between my PV 23kW DC and the inverters putting out 15.2kW AC. I still don't really understand that like I thought I did. I thought I understood it, but clearly I do not. :\

 

[wattmatters]

The other thing I'm figuring out is the stark difference between my PV 23kW DC and the inverters putting out 15.2kW AC.

It is common with grid-tied PV systems, indeed it's generally good practice, to install a PV array with a peak output rating higher than the AC output rating of a grid-tied PV inverter.

Most GT PV inverters will have a technical limit on this "over panelling" ratio. I'm not sure what that limit is for your specific inverter model is but it's typically in the 1.3 to 1.5 range.

Since 23 / 15.2 = 1.51 then I'd say you inverters are rated at the upper end of that range and the installers have maxed out that DC to AC ratio. There can also be some bureaucratic limitations on that ratio (e.g. like here in Australia STC credits are limited to systems with a max DC:AC ratio of 1.333), so for clarity I'm only talking about the technical limit set by the inverter manufacturer.

So if it's the middle of a nice sunny day and the PV arrays are capable of generating, say, 23 kWh, then the inverters will cap their AC output to the 15.2 kW max they are capable of supporting. The panels are not actually producing 23 kW, they are only outputting the power the inverter is drawing from them. The inverter manages this process.

You might think that losing all that possible production capacity sounds pretty lousy, but consider that for the vast majority of the time the PV array will not be capable of generating at its peak power output, but rather at some fraction of that.

Earlier in the morning and later in the afternoon, when it's cloudy or raining, in the Winter half of the year when the sun is lower in the sky, if the PV arrays face multiple directions and are not all facing the Sun at the same time, and so on. In these scenarios the PV array will generate a lot more energy than a smaller array (and likely it will still be less than your inverter's capacity) and as a result the capacity utilisation of the inverters will be much higher. They will operate at a higher output, for longer.

Ideally you will have your PV arrays facing different azimuths (perhaps even at different tilt angles), e.g. some East, some West, some South (assuming you are northern hemisphere). This will provide a nice wide, flat solar PV production curve better suited to the over panelling ratio you have. But generally we are limited by the roof we have to work with. Sometimes there is choice on which section of roof to use.

So, sure, in Summer your system might clip production during the hours either side of solar noon, but at other times of day and year and when the cloud is hiding the sun you will be very grateful to have the extra PV capacity to capture those far more scarce photons and keep your batteries charged to power you through the night.

 

[turtle2472]

Thanks again for the details. It is helping me truly understand this.

So should I buy one additional Powerwall and install it. I would then have a max input to the Powerwalls of 22.8kW with continuous being 15kW. Since my inverters top out at 15.2kW AC they would be able to max charge the three Powerwalls at 15kW.

In that case, I would also be able to upgrade my inverters to 10kW each pushing 20kW because the three Powerwalls would be able to handle this. Am I correct in this?

I would have less clipped PV production and then be more limited to the other environmental factors because my "over paneling" ratio would go down to 1.15. Still more than minimal but not a lot of degradation or "loss" allowed to maintain max production. It would be more than I'm making now, but still less often that I will max out at the inverter output. Where now I stay maxed for the bulk of the daylight hours. Do I have this right too?

 

[wattmatters]

I can help you understand how they work but I can't tell you what you should do.

To me 3 x Powerwall 2s seems like expensive overkill, but then I have zero idea about what it is you are trying to achieve, budget, expectations on what you are trying to power etc.

Powerwalls are not really a DIY thing.

 

[zanydroid]

I agree installing PowerWalls feels bad in this scenario, and it's hard to see what to recommend. While I may have an overly cheap bias, I have wasted a lot of time analyzing the ROI for California Net metering.

Shooting from the hip:

• I think the incremental value from going from 2 to 3 PowerWalls is probably not that good. You already have backup power capability. (However you haven't expressed your system goals and requirements, so there might well be a capacity break point where you get a lot of value on the additional storage).
• Adding a PowerWall just to avoid clipping feels like a really expensive way to increase generation.
• Have you done a cashflow/ROI analysis on this? I joke that I've wasted a lot of time on calculating the value of PowerWalls, but doing that is a VERY effective way to convince yourself not to do powerwalls and play with other stuff instead. And you don't even need to be that good at financial modeling.

I'll add that there are probably other ways to avoid the PV AC / Powerwall AC problem. Namely, you could add another 5kW string inverter. Then put a 2-pole contactor/relay that automatically opens in a grid-down scenario. Now you are following Tesla's AC coupling system sizing restrictions. Granted, this will mean some production capacity is "stranded" in a grid-down scenario, but you'll still make more in grid-up. The other downside I see here is that these kind of custom PLC approaches are hard to explain to an installer. Actually you probably don't even need a PLC. Just land the 5kW inverter on the opposite side of the Powerwall ATS from everything else.

 

[turtle2472]

I do understand that Powerwalls is not a DYI thing. There is a ton of back story I'm avoiding sharing at how I ended up where I am now with my current system. Long before I even conceived of a DYI solution I ended up in a contract not realizing how I could build my own (with technical help from pros) rather than paying an installer/dealer for everything. I joined here during a period where I thought my contract had fallen apart and began digging for new options. Yes, there are changes I would make to what I have now, but contracts were already signed and the company hadn't gone under when I realized there were other options. I am overall happy with what I have now though feeling my limitations.

So that that being said, I'm not asking if I should add another Powerwall but rather do I understand the overall increase in the system. I do agree that adding another Powerwall and upgrading the inverters to avoid clipping doesn't make sense financially, but I'm checking my understanding of the system and capacity/limitations.

My original goal was prepper like and completely off grid. That became unrealistic for my home situation so we (Mrs T and I) agreed with grid tied but energy storage to allow for grid down (or turned off) with minimal interruption to our lives. We are on net metering (1:1) until 1 Jun 2029 when we get moved to Solar Time of Use. So the idea being that by the time we lose net metering we have a solution that allows for complete independence. Our power bill should be a net gain going forward until the demise of net metering. At that point we will have to pay for grid access, and usage during dark times for anything not covered by the batteries.

Of course, financially speaking, it doesn't make sense to buy any more batteries or any other component for our PV/Storage systems for that matter. We did gain a level of independence and that has tangible value to us.

 

[zanydroid]

OK that context is very helpful b/c TBH it makes us feel better if the request is logically coherent with the assumptions and goals of the system. For a thought experiment I'll indulge everything. For something that sounds like flushing a few thousand dollars down the drain, I'll go into "Please don't do that" mode.

If you go off net metering with solar then batteries have a better ROI.

IMO listed ESS will go down in price by 2029 with the expansion of ESS market in California. Unless NEM3 completely kills rooftop solar (possible). I don't think it makes sense to put hardware in right now on the storage side. StorageEdge inverters are pretty plentifuil on eBay so you could consider reducing the overpanel ratio with DIY, without much investment (if you are clipping now).

But you have to be smart about staying within the limits -- IE having smaller AC size and KWh on the battery/inverter side comes with extra design / operational responsibility on your part to protect the equipment.

With StorageEdge you are kind of locked into their inverters b/c of the optimizers, unless you want to unmount and sell all of those to go with non-StorageEdge. So AC coupling is likely going to be the path for a DIY friendly, lower cost battery approach. At least you have the choice of swapping to other SolarEdge inverters that may have better AC Coupling behavior, without having to swap modules (I don't know if these exists, this is a theoretical possibility).

 

[wattmatters]

A good way to deal with energy supply limitations is to reduce the energy capacity required.

Too much effort and resource is often spent on supplying energy when often the better option is to work on ways to reduce the need for the energy consumption in the first place.

23 kW of PV, 15 kW of inverter and 27 kWh of storage is not small capacity for a home. What on earth do you absolutely need to have operating during a grid outage? Surely the sort of capacity required for a grid down scenario is not anywhere near that much?

 

[turtle2472]

Oh, we have taken MANY steps to reduce consumption. Basic things like LED bulbs throughout instead of incandescent or CFL. Replaced some older appliances for new efficient ones. Added smart switches to help the kids turn off the lights since they can't seem to recall how to do that. Those kinds of things.

For the big things though, that is where the Span Panel comes into play. It categorizes loads into Must Have Circuits, Nice to have circuits and non-essential circuits. If the grid goes down the non-essential loads are shed. Once the battery capacity is reduced to 50% then the nice to haves are shed. All duel pole breakers are non-essential (climate control, stove, sub panels etc...). Since the Span is limited to 32 slots we have sub panels off of it and those are shed with non-essentials. So the must have circuits are limited to refrigerators, "critical lighting" and outlets that are deemed essential which is only a handful.

We went with the Span Panel as opposed to moving circuits to a separate "critical loads" panel because we wanted the option to be able to programmatically move any of the sub panels into must have or nice to have. With a specific critical loads panel we are unable to fluctuate without electrical rework. This allows more electrical loads to be able to be powered by the Powerwalls without going over capacity on discharge.

Under "normal" usage conditions the two Powerwalls will power must haves for more than a day if the grid goes down.

The interest in a larger storage capacity goes back to the prepper mindset of being able to be completely independent from the utility company. It isn't a priority right now, but it is something we are planning for. Mrs T and I like to think of ourselves as "casual preppers".

 

[solar8484]

It appears financial ROI doesn't matter for you so if financial means allow then you should get a Ford F150 Ligtning with the 100+Kwh battery and the Ford Intelligent Backup option with 10kW additional DC coupled solar panels. You'll probably be able to go off-tide for weeks.

 

[wattmatters]

The interest in a larger storage capacity goes back to the prepper mindset of being able to be completely independent from the utility company. It isn't a priority right now, but it is something we are planning for. Mrs T and I like to think of ourselves as "casual preppers".

That implies cutting the grid cord and going completely off-grid. IMO Tesla are the wrong product for going off-grid.

But if you mean remaining connected to the grid while all-but eliminating the import of energy from the grid, then Powerwalls are a good product for that.

I thought Tesla stopped selling Powerwalls which were not part of a full Tesla solar PV package.

 

[turtle2472]

Yeah, I'm gathering that. If I had found Will's videos and this forum before I had already signed contracts I'm 100% positive I would have a very different build. I'm still here and have been reading because I'm enjoying the topic and wanting to know more about it and be able to speak more intelligently about it. Plus, I don't like that fact that I was ignorant of so much about these systems that I signed contracts "blind" on faith that the installer/dealer knew best. The specs looked good on paper, and this system does generally do what I want so I don't really regret it, I know I could have designed better and cheaper. :\

The going off grid part was something we wanted, but put on the back burner because with the options available from the installers we talked to there just weren't many. Mind you, none of them went into server rack batteries or anything beyond their favorites to sell like Tesla, LG, etc... (Speaking of, my dealer says they still sell Powerwalls but they have a very long lead time on orders right now.)

So at this point we have what we have and I'm trying to understand it better. That will enable me to make my own choices going forward and how to best evolve my system as time marches on.

For now, yes we do intend to stay grid connected with the intention of shifting our usage habits to be better about consuming electricity while the sun is up and minimize electrical consumption during dark hours. Depending on how the energy bill looks once we are shifted to Solar Time of Use will determine how much stress we put on getting severed from the grid. I don't envision us ACTUALLY turning off our grid service, but rather working to reduce consumption to 0%.

Edit: For a little additional context, I signed the contracts on my build November 2021! We got permission to operate 9 March 2023. This has been a very long time coming and we were on hold due to a meter base changeout and that availability.

 

[wattmatters]

You have a nice system, I would certainly be happy with it. Combined with the SPAN panel, and given the earlier issues of system scope we discussed, it sounds to me like a quite a well considered set up.

I don't know how much these systems (should) cost over there but provided you are OK with the outlay then you should be satisfied. Sure you could have built your own for less but I doubt it would be as polished, and assuming the installer is a quality outfit they should be around to support you if ever that's needed.

Depending on how the electricity market operates where you are, there is even future potential for becoming part of a virtual power plant.

But for now, with 1:1 net metering (which is an insanely generous deal frankly - although means the battery doesn't contribute much to your bill reduction) then I think you will find you'll have very low bills.

The Span panel I'm sure can be programmed to help with load management beyond managing essential loads during outages.

You have well built kit.

Frankly I think going completely off-grid isn't generally a good idea - the grid is a public service and the more who remain connected the better it is for everyone. But I understand that in some areas the power supply isn't that great (or even available at all), or the power company isn't particularly consumer friendly, or they charge like wounded bulls. At worst, the grid makes for a fantastic backup generator!