Need Advice: Uses for a PowerWall 2 in a solar home with an existing whole-house backup generator?

[Jim Holder]

Hey all,

We won a PowerWall 2 for making five referrals and are looking at various ways a PowerWall 2 can be useful to us. Our utility has Net-Metering and also does not bill based on time of use.

Our home already has a 20 kW natural gas whole-house backup generator (Briggs & Stratton) plus we have a 22.5 kW solar system on the roof (60xSunPower 360's), hence don't see a practical use for a PowerWall 2 other than maybe smoothing out occasional brown outs(?).

I thought of one option to configure it to fill/top off each day from solar generated power and use that to charge our Tesla Model S at night (the local Tesla Energy rep said this setup is doable), but since we have net-metering with our utility company where any excess power generated goes back to our utility and winds our meter back, it seems to be the same difference drawing charge power from the grid as it'll just use up any credits of power we might have pushed that day back to the utility. So that's not really a good use for it.

Any other ways a PW2 can be leveraged in such a setup?

TL;DR: Are there other ways to leverage a PowerWall 2 in a solar home with an existing whole-house backup generator (other than asking for credit on our Tesla account instead to be used towards other Tesla car purchases)?

Thanks!

 

[vloschiavo]

Other uses:
-reduce or eliminate reliance on dirty grid power during low solar production times: night, rain, snow, etc.

Random thoughts:
-The powerwall isn't going to charge your car very much. If you are only getting one, that's < 14kWh of storage. Your car is anywhere from 60-100kWhs. Instead use it to power your home / time of use load shift.

 

[miimura]

The Powerwall would greatly reduce the amount of time that you would need to run the generator. However, 22.5kW of solar is a lot for one Powerwall. Two Powerwalls would be better and it would also allow you to back up larger loads.

 

[Jim Holder]

Other uses: Reduce or eliminate reliance on dirty grid power during low solar production times: night, rain, snow, etc.

Thanks for the quick reply. With net-metering I can accomplish the same: Our solar pushes clean excess solar power back into the grid during the day and it's just returning it at night.

-The powerwall isn't going to charge your car very much. If you are only getting one, that's < 14kWh of storage. Your car is anywhere from 60-100kWhs. Instead use it to power your home / time of use load shift.

Good point re capacity maxing @ 14kWH. Forgot about that. We run through about 18-20kWh/month, so about 630 kWh's/day. This would give us about two days of juice. But we already have a 20kw backup generator. Grrr.

 

[Jim Holder]

The Powerwall would greatly reduce the amount of time that you would need to run the generator. However, 22.5kW of solar is a lot for one Powerwall. Two Powerwalls would be better and it would also allow you to back up larger loads.

Interesting point. We are getting only one PW from the referral reward program. With an existing 20kW backup generator it doesn't seem to make financial sense to pay another ~$7k for a 2nd PW.

 

[miimura]

Interesting point. We are getting only one PW from the referral reward program. With an existing 20kW backup generator it doesn't seem to make financial sense to pay another ~$7k for a 2nd PW.

You seem to have your answer already. If it doesn't bother you that your solar is idle and you're burning fossil fuel when the grid goes down, then just leave it alone. Tesla's integration with generators is poor anyway.

 

[Jim Holder]

You seem to have your answer already. If it doesn't bother you that your solar is idle and you're burning fossil fuel when the grid goes down, then just leave it alone. Tesla's integration with generators is poor anyway.

We have a 200 amp service. Would the PW2 be able to shoulder that kind of load if the grid went down?

 

[miimura]

We have a 200 amp service. Would the PW2 be able to shoulder that kind of load if the grid went down?

The size of your service doesn't really tell us that much about your loads. I have 320 amp service and two Powerwalls. In two months I have yet to exceed the power limit of one Powerwall (5kW). My central heating, water heating and clothes drying is on natural gas, my EVSEs are not backed up, and I haven't used A/C.
You really need to inventory your larger loads to get an idea how Powerwalls will integrate into your home. Startup loads from things like A/C compressors are also an issue when running in backup mode.

 

[Jim Holder]

The size of your service doesn't really tell us that much about your loads. I have 320 amp service and two Powerwalls. In two months I have yet to exceed the power limit of one Powerwall (5kW). My central heating, water heating and clothes drying is on natural gas, my EVSEs are not backed up, and I haven't used A/C.
You really need to inventory your larger loads to get an idea how Powerwalls will integrate into your home. Startup loads from things like A/C compressors are also an issue when running in backup mode.

Thanks for pointing that out. I guess it's worth to have an electrician do an audit to see what our regular and peak loads look like so we can work from better data.

Would you happen to know what the amp output ceiling is on a single PW2?

 

[miimura]

Thanks for pointing that out. I guess it's worth to have an electrician do an audit to see what our regular and peak loads look like so we can work from better data.

Would you happen to know what the amp output ceiling is on a single PW2?

Each Powerwall 2.0 AC unit is rated for 7,500W peak, 5,000W continuous. At 240 VAC, that's about 21 amps continuous. I've heard that Tesla's design guidelines include a restriction that any load circuit larger than 30 amps per Powerwall may not be backed up. So, if you have a 40 amp circuit for a central A/C unit, that load will not be allowed on the backup side of the gateway served by a single Powerwall. A system that includes two Powerwalls would exclude circuits larger than 60 amps.

 

[CA-Gbm84]

For your sizing
- There is a hard limit of 7.6 kW-AC of solar per Powerwall. This is relevant when the grid goes down and a large solar system is putting a lot of power into Powerwall (which can accept 5kW). You can either split your solar into two inverters (one up to 7.6kW) and only have that within the backup circuit or add 2 more Powerwalls.

For your load
- Usually you can get that from your utility - find the max usage in kWh per hour will give you the kW. But you should be fine the gateway that controls connects Powerwall to the grid can handle up to 200A.
- The only consideration is the large loads (>30A) like the EV charger or AC, which may also push you to more Powerwalls.

For your reasons to get it
- 22kW of solar is a lot, would be interesting to see just how much is actually being used on site. Yes net metering provides an some of that value to you, but those electrons coming back at night are almost certainly not from solar. Solar generation is good, solar consumption is better.
- It's Free! Even if you buy another one you're adding $10k of value to your home and paying $5k. Even without TOU (which will be coming) thats a pretty good return plus you reduce the usage of the generator.

 

[power.saver]

There is a hard limit of 7.6 kW-AC of solar per Powerwall.

Where does this 7.6 kW come from? I can't find it in any spec, and was told the max solar is 5.0 kW per PW2.

 

[CA-Gbm84]

From the Tesla website, I guess the charge rating is different to the discharge rating

Solar Integration | Tesla

 

[power.saver]

interesting. My advisor said 5.0 kW was the limit per PW2. I suppose the 7.6 kW accounts for less than ideal solar production or up to 2.6 kW typical on-site load.

The thing is, if solar ever exceeds what the PW2 can accept minus on-site load during an outage, it is going to shut down the solar production. Seems you would want to size accordingly.

 

[Cottonwood]

interesting. My advisor said 5.0 kW was the limit per PW2. I suppose the 7.6 kW accounts for less than ideal solar production or up to 2.6 kW typical on-site load.

The thing is, if solar ever exceeds what the PW2 can accept minus on-site load during an outage, it is going to shut down the solar production. Seems you would want to size accordingly.

One of the shortfalls of the Tesla PW is that its control of Solar PV Inverters is only On/Off when disconnected from the grid. This is good because it makes the PW compatible with almost any grid tied inverter. If the PW needs solar when disconnected from the grid, then the PW makes a clean AC waveform and the Solar Inverter produces. When the PW does not need (or can't take the output) of the Solar Inverters, then the PW makes an out of spec to the Solar Inverter, but acceptable to the loads, AC waveform, and the Solar Inverters shut down.

The SMA Sunny Island Inverters can control the SMA Sunny Boy Solar Inverters proportionally with a proprietary protocol. This is nice, but the Tesla PW implementation is more general.

While the web page says 7.6 kW, using the more conservative 5.0 kW of Solar PV per PW is probably the more conservative way to go. Most Solar systems are rated as DC Watts produced by the panels under ideal conditions. The best Solar Inverters have an efficiency of 90% or slightly greater. 90% of 22 kW DC is 20 kW of AC. Given that there are almost always a few AC loads, I would propose that 4 PWs would be a reasonable minimum against a 22 kW Solar System.

Good Luck!

 

[NuShrike]

Given that there are almost always a few AC loads, I would propose that 4 PWs would be a reasonable minimum against a 22 kW Solar System.

If you an AC PV system, you can always move some of the PV breakers to your main power-panel instead of the Tesla Gateway subpanel.

This means only the % of your full PV system behind the Gateway is available in off-grid situation, but it'll match less PW2s better.

For example, if only 10 kW of your AC solar system is on the subpanel, then that'll perfectly match 2 PW2s.

 

[Jigglypuff]

Since generators and PWs aren't really compatible, I think you'd need to go from line to powerwall on a branch separate from the branch the generator is on, with the PW's own loads. Doesn't seem worth the bother, really. You can, however, drop the generator and replace it with multiple power walls. That's essentially what we're doing. I'm sick of maintenance.