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Is there a electrical reason for two+ PowerWall 2s vs PV system size?

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Hello, I'm in the permitting phase for a 9.72 kW PV system installation.

At the same time, I've been getting quotes for the PowerWall 2. My goal is to have a grid-tied system AC connected to the PW2s, that can still charge and use the PWs when the grid goes offline.
  • Telsa says I need two PWs because 9.72 kW dumping 40.5A (@ 240v?) is too much for one PW.
  • Infinity Solar, which also does PW installations, says I can get by with one PW.
They both can't be wrong, as neither would install a faulty system, but Tesla could be up-selling.

Now I've seen a number of forum numbers with large PV systems whom are getting by with one PW. I.E. ecarfan has a 9.8 kW system with one PW1. Shygar has a 7.5kW system with one PW2.

So does anybody else know exactly why we can't just have one PW2 for grid and off-grid?

Meanwhile, my AC is on a 50A breaker, but the label says it only needs a max 35A breaker. I'll need two PWs to back-up the AC?
 
First, ecarfan has a first generation PowerWall, so the solar sizing vs. PowerWall sizing considerations are different. The SolarEdge inverter is doing all the work and it can curtail the solar itself.

Second, the PowerWall 2 can only absorb 5kW of power, so it would not be stable if your solar was putting out, for example 8kW. If you have more than one inverter, you could wire some of the solar outside the Gateway so that it is shut down when the grid is down. However, then you would not have that generation capacity available during an outage. Two PowerWalls is really better for a solar array over 6kW.

Third, when the batteries get full, the PowerWall does not yet have elegant solar curtailment when off-grid. There is certainly room to improve this, but right now people have shown that most solar inverters will not curtail until the frequency goes out of range, then they completely shut down. They will periodically check the frequency and if it's back within limits, then it will start generating again.
 
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Third, when the batteries get full, the PowerWall does not yet have elegant solar curtailment when off-grid. There is certainly room to improve this, but right now people have shown that most solar inverters will not curtail until the frequency goes out of range, then they completely shut down. They will periodically check the frequency and if it's back within limits, then it will start generating again.
My system will be micro-inverters for 27 panels.

Okay, so one way is to ask my installer to split the system into two breakers. Something like 18x (6.480 kW) on the PW side of the gateway, and 9x (3.24 kW) on the service-side of the Gateway, cool.

So even if there's PW2s and they fill up, still same problem. So the PW2s will probably shift the AC frequency and force the micro-inverters to shut down?
 
My system will be micro-inverters for 27 panels.

Okay, so one way is to ask my installer to split the system into two breakers. Something like 18x (6.480 kW) on the PW side of the gateway, and 9x (3.24 kW) on the service-side of the Gateway, cool.

So even if there's PW2s and they fill up, still same problem. So the PW2s will probably shift the AC frequency and force the micro-inverters to shut down?
There is a relatively low maximum number of micro inverters on one breaker anyway. A quick look at an Enphase datasheet says they use 20A breakers and you can't have more than 3,840W on each breaker, so you would probably have three strings of micros.

Yes, the PW2s slowly shift the frequency and it depends on how the inverters respond to that. In theory, they could gradually curtail the solar as the frequency slowly shifts away from 60Hz, but in the worst case, the PW2 will just keep shifting the frequency and then they will do a complete shutdown when it goes out of their allowed range.
 
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Yes, the PW2s slowly shift the frequency and it depends on how the inverters respond to that. In theory, they could gradually curtail the solar as the frequency slowly shifts away from 60Hz, but in the worst case, the PW2 will just keep shifting the frequency and then they will do a complete shutdown when it goes out of their allowed range.

The data sheet for the IQ6 shows a configurable roll off in power output based on line frequency.
 
You'll need to check the LRA current for the compressor and fan. The startup draw is much higher than the steady state. I read there is an easy start kit they can install to make it more compatible.
LRA is 105. This is an ancient A/C that I'll replace next. So I'll need the "starter kit", thanks.

There is a relatively low maximum number of micro inverters on one breaker anyway. A quick look at an Enphase datasheet says they use 20A breakers and you can't have more than 3,840W on each breaker, so you would probably have three strings of micros.
Yes, looks like 20A breakers with max of 12 panels per breaker: Equinox.

I'd assume because of the NEC 690.12 / IEEE 1547 conformance, the inverters will shut off when out of phase. Checking with installer what the actual behavior is.

So summarizing, the preferred config is two PW2s to stably absorb the power-generation when off-grid, but one PW2 can work as long as the excess power is on the other side of the Gateway. Curtailment is dependent on the micro-inverter.

Question answered, thanks!
 

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LRA is 105. This is an ancient A/C that I'll replace next. So I'll need the "starter kit", thanks.

Yes, looks like 20A breakers with max of 12 panels per breaker: Equinox.

I'd assume because of the NEC 690.12 / IEEE 1547 conformance, the inverters will shut off when out of phase. Checking with installer what the actual behavior is.

So summarizing, the preferred config is two PW2s to stably absorb the power-generation when off-grid, but one PW2 can work as long as the excess power is on the other side of the Gateway. Curtailment is dependent on the micro-inverter.

Question answered, thanks!

:)
Yeah, that AC was 10 SEER back in 99. You can get a new system that is 40% more efficient for 2k or so (materials). There are also (pricier) heat pump units that play nice with solar.
 
:)
Yeah, that AC was 10 SEER back in 99. You can get a new system that is 40% more efficient for 2k or so (materials). There are also (pricier) heat pump units that play nice with solar.
Oh, I'm definitely moving to a high seer HP next.

Been learning a lot about home equipment recently, and I'm just glad everything is still working long enough to get upgraded.
 
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Hello, I'm in the permitting phase for a 9.72 kW PV system installation.

At the same time, I've been getting quotes for the PowerWall 2. My goal is to have a grid-tied system AC connected to the PW2s, that can still charge and use the PWs when the grid goes offline.
  • Telsa says I need two PWs because 9.72 kW dumping 40.5A (@ 240v?) is too much for one PW.
  • Infinity Solar, which also does PW installations, says I can get by with one PW.
They both can't be wrong, as neither would install a faulty system, but Tesla could be up-selling.

Now I've seen a number of forum numbers with large PV systems whom are getting by with one PW. I.E. ecarfan has a 9.8 kW system with one PW1. Shygar has a 7.5kW system with one PW2.

So does anybody else know exactly why we can't just have one PW2 for grid and off-grid?

Meanwhile, my AC is on a 50A breaker, but the label says it only needs a max 35A breaker. I'll need two PWs to back-up the AC?

Both are correct. My PV system was large enough that Tesla wanted to sell me 3 Powerwalls instead of the 2 that I wanted.

Tesla is right: Under full sun with no loads your PV system can generate more power than what the PW can sink/absorb. When this happens the Powerwalls will effectively shutdown the PV system so the system won't charge in situation. This could be true even if the batteries are completely empty.

Infinity Solar is right: The conditions under which the system will not charge is likely rare. You won't care that the system is not charging because you are probably not home and there are no loads. The situation will self correct, when peak sun passes or as load increases. You can manually correct this by disabling some of your PV system. If your inverters are able to vary their output with frequency then this can be avoided.

arnold
 
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I'm getting feedback from Tesla Energy that their "SureStart" adapter doesn't support "reciprocating compressors". Anybody heard of this?

I've found this similarly named SureStart adapter which explicitly says it does: http://hypereng.com/literature/SC5600HM-SureStart-Specification-Catalog.pdf

Otherwise, TE wants me to get 4 PowerWalls in order to support my 105 LRA. I'm not on-board with that, especially as it's way over the SGIP limit.
 
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I'm getting feedback from Tesla Energy that their "SureStart" adapter doesn't support "reciprocating compressors". Anybody heard of this?

I've found this similarly named SureStart adapter which explicitly says it does: http://hypereng.com/literature/SC5600HM-SureStart-Specification-Catalog.pdf

Otherwise, TE wants me to get 4 PowerWalls in order to support my 105 LRA. I'm not on-board with that, especially as it's way over the SGIP limit.
It would be cheaper to replace your compressor/condenser unit with a higher SEER unit than to buy the extra PowerWalls.
 
I'm getting feedback from Tesla Energy that their "SureStart" adapter doesn't support "reciprocating compressors". Anybody heard of this?

I've found this similarly named SureStart adapter which explicitly says it does: http://hypereng.com/literature/SC5600HM-SureStart-Specification-Catalog.pdf

Otherwise, TE wants me to get 4 PowerWalls in order to support my 105 LRA. I'm not on-board with that, especially as it's way over the SGIP limit.

The unit you posted has a max starting amperage of 65, so it can't start your AC unit.
 
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The unit you posted has a max starting amperage of 65, so it can't start your AC unit.
? Page 5, it gives a chart, where it reduces a 109A in-rush to a 33A in-rush. So that looks promising.
Page 9: Model 064 can handle starting amps of 118A.
Ah, page 15, the SS1B16-32 max amps is 65A.

Meanwhile, the 3-phase has 150 max starting amps: http://hypereng.com/literature/SC5630EH.pdf

So then I'm mystified why it would bother describing > 100A support in the single-phase literature. I'm poking their sales email to clarify.

It would be cheaper to replace your compressor/condenser unit with a higher SEER unit than to buy the extra PowerWalls.
Very true. But even cheaper to get a sure-start retrofit until everything else gets installed, I would prefer. I don't want to inflate a PV + PW2 install into a simultaneous HVAC upgrade.
 
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They told me I couldn't install with my mini inverters on every panel?

There needs to be a method to disable the inverters if their output is greater than the demand and charge rate of the PW when in isolation mode (no grid). Some micro-inverters can be configured to reduce output based on frequency. If the PW can change output frequency based on voltage/ charge rate, then the parts may be able to co-exist.

It seems like Tesla's preferred set up is to have a a certain max PV capacity per PW on the house side of the gateway. With per-existing/ wired micro inverters, this may not be possible.