Emergency Power PWs Shutdown Due to PV Overproduction

[BIC1]

I may be wrong on this but I understand Tesla requires each PW2 to have 7.5 kW of PV to keep the the system from shutting down due to inability to unload excess power to the grid. I have a micro-inverter setup with 38 panels.

My PV system puts out an actual peak of 10 kW (12.1 kW advertised). I initially had 1 PW2 installed and while testing system found out that it will shut down on a bright sunny day because the PW & house can't absorb all the energy. I am now adding a second PW2. Isn't it still possible for 2, 3 or more batteries to shut down if they are full and the sun is shining?

It seems more batteries won't prevent a shutdown on sunny days, is this correct? Thanks.

 

[jjrandorin]

More batteries will prevent the problem you were talking about. While its possible for "2, 3 or more batteries to shut down if they are full and the sun is shining" thats not the same thing you are talking about.. You mean "sun is shining and we are off grid" in which case whether they will shut down or not would depend on how much power your PV system can generate.

There would be zero chance of your system shutting down if you had 3 powerwalls, since it generates a max of 10kW and each powerwall can take 5kW sustained input. Your system doesnt generate 15kW so you wouldnt have an issue with 3. Whether you have an issue with 2 or not depends on whether its actually 10kW your system puts out, or 10.4k or 9.9kW at its peak sustained input.

Whomever did your install should have informed you that 1 powerwall was not enough for that much solar, unless it was configured in such a way that part of it would not run at all when off grid.

 

[Matt-FL]

If I am in off-grid mode, with 20.4kW system and 4 PWs, when the batteries fill the solar will shut down until a (Tesla known) battery % is reached, then solar turns back on. This only happens though in off-grid situations. When on-grid, the excess is sent to the utility.

 

[BIC1]

@jjrandorin , @Matt-FL , Yes, I do mean off grid. Thanks for the clarification.

Peak outputs are:
2019 - 10,031
2020 - 10,031
2021 -- 9,993
2022 -- 9,992
2023 -- 9,990

These numbers are from the APsystems app. Looking at the Tesla app for 07/17/2023, the day AP says 9,990; Tesla is about 8,500-9,000, estimating from the graph, below ; I don't see a way to get a precise number from the Tesla app. I compare monthly solar energy totals and the AP app always shows 5-6% more energy generation than Tesla does. I'm guessing the PWs determine shutdown from the Tesla numbers, so that seems to safely put me under 10 kW peaks. Not to mention reduced generation as each year passes.

What my installer should have told me? A story for another day. Thanks.

 

[arnolddeleon]

More batteries will prevent the problem you were talking about. While its possible for "2, 3 or more batteries to shut down if they are full and the sun is shining" thats not the same thing you are talking about.. You mean "sun is shining and we are off grid" in which case whether they will shut down or not would depend on how much power your PV system can generate.

There would be zero chance of your system shutting down if you had 3 powerwalls, since it generates a max of 10kW and each powerwall can take 5kW sustained input. Your system doesnt generate 15kW so you wouldnt have an issue with 3. Whether you have an issue with 2 or not depends on whether its actually 10kW your system puts out, or 10.4k or 9.9kW at its peak sustained input.

Whomever did your install should have informed you that 1 powerwall was not enough for that much solar, unless it was configured in such a way that part of it would not run at all when off grid.

I believe rapidly changing loads/production can also trigger a PV shutdown. This is also influenced by whether the PV system OP has curtailment features or it is binary on/off.

But under normal circumstances (Powerwalls not full, ordinary operating temperatures, relatively steady load, etc.), three Powerwalls can absorb all the production from the OP's PV system. Practically speaking, two would likely be sufficient as well.

 

[Frontrange PV]

Hmmm... Arnold DeLeon brings up a good point.

There's a thing called "frequency shift curtailment" whereby the Powerwall 2 or the backup gateway 2 (not sure which, I suspect the gateway) changes the frequency of your system while in off-grid mode in order to control how much power your inverters are making. This may not be supported by all inverters, but it basically means that there isn't a set ratio that you need to observe in order to maintain power in an off-grid scenario.

So if you're configured for frequency shift curtailment, and making more solar power than your battery can use for charging, and your house load doesn't make up the difference, and the grid isn't available to absorb the difference (that's 4 conditions that have to be met), then the system raises the AC frequency progressively above 60 Hz. The inverters respond to the higher frequency by reducing their output, until they eventually shut off. At that point, your house is running on the Powerwalls(s) alone, until they discharge far enough to call for a charge. When that happens, the system reduces the AC frequency again, the inverters come back on line, and provide enough power to both run your house and charge your battery, provided that there's enough solar available to do both.

In theory, if you have enough Powerwall capacity to carry your home through the dark hours, and have enough solar to both run your house and recharge the batteries, you can run more or less indefinitely until the grid comes back, without resorting to a whole house backup generator. In practice, this gets pretty hard in December and January because there just aren't enough sunlight hours in the day during that time of year in North America.

Our system is built to cover all of our typical usage other than charging the car or running the air conditioner (i.e., for baseline needs) 10 months out of the year. We're net positive 8 months of the year, and break even in November and February, based on average daily consumption. Our baseline needs are small enough that 1 Powerwall can carry our home under normal circumstances for 12-16 hours, so we only bought the one.

YMMV, of course.

The devil's in the details as they say... But there are other options aside from those noted above. It's not just about how many Powerwalls you have relative to your peak solar output as I understand matters.

 

[jjrandorin]

I think for curtailment, the inverters have to support it and there needs to be a profile for it. My system doesnt support curtailment (inverter from late 2015). Some do, and some dont, for sure. I am not sure what the line is for that.

 

[BIC1]

My PV was installed in 2019. Unfortunately, at the time, Enphase were difficult to get so I got AP YC500A micro-inverters. Didn't realize until I later discovered the inadequacy of one PW about the frequency thing. My micro-inverters do not have the ability to progressively raise frequency as did the Enphase IQ7 or the follow on AP YC600.

My so-so recollection is that I had Tesla raise the PW frequency from 60 to 62. I don't think there is any way for me to verify that was done.

I was supposed to have 2 more PW (3 total) installed tomorrow. Another long story, but I'm only getting 1 tomorrow. Whether I'll get the second before the rebate expires is unlikely. Powerwalls are solely for emergency backup.

I'm wondering if I should just stick with 2 total PWs anyway since my PV makes slightly under 10 kW. Reasoning includes that PW2 is now a somewhat obsolete technology with home storage moving away from lithium-ion. Having several hundred pounds of Lithium-ion in the middle of my basement does make me just a wee bit nervous.

I also have a Gateway 1. Maybe in another 10 years it'll be worth switching to a newer technology. Might also get an EV with 2-way charging (V2H). Not sure if those integrate with Powerwall.

 

[jjrandorin]

My so-so recollection is that I had Tesla raise the PW frequency from 60 to 62. I don't think there is any way for me to verify that was done.

More than likely, what you are remembering here is you would have asked Tesla to drop the frequency from 65 to 62, to stop the 65hz frequency that the gateway sends out by default from tripping some of your electric devices like small home battery backups, microwaves, etc.

 

[BIC1]

More than likely, what you are remembering here is you would have asked Tesla to drop the frequency from 65 to 62, to stop the 65hz frequency that the gateway sends out by default from tripping some of your electric devices like small home battery backups, microwaves, etc.

Yes, more than likely. Thanks for the memory refresher. But I am certain my micro-inverters are on/off, no easing off the power.

 

[jjrandorin]

I'm wondering if I should just stick with 2 total PWs anyway since my PV makes slightly under 10 kW

Given what you have shared, I would likely go with 2 instead of 3 if it were me, if (big if) 2 powerwalls started everything I thought I might want to start, in a power outage (sump pumps, pool pumps, air conditioner etc). I dont know your current setup but motors can sometimes take a lot of current to get started (more than they take to keep running). If everything started with 2 (or at least everything I wanted to start), then I would go with 2. Otherwise I would get the third one.

Whether tesla powerwalls are ever going to work with anyones vehicle to X, is debatable and its not something I would plan for, just be happy if it ever happened.

 

[BIC1]

@jjrandorin , yes, I can start everything with 2 PWs. I have 2 AC, both with soft-starts installed. 99% of the time, I only use the upper level one anyway. So, 1 soft-start AC, a sump pump that sees light action and no other big startups.

The main use for a third PW would be winter, which has half the daylight as summer along with occasional snow covering the panels for a few days and more rarely, maybe a week. We did have several consecutive overcast days at least once this summer. But yeah, 2 is adequate, 3 a luxury.

Not to mention, in the 5 years in current house, we get several outages a year, usually lasting minutes. Had one or two outages that maybe went an hour. By the time I shut almost everything off in the house, the grid comes back.

On the other hand, at previous metro house 15 years ago, ice storm knocked power out for two weeks. Installed a Generac NG after that, but never really needed that with occasional one hour outages.