3-4kW System w/ 2 Powerwalls?

[supralover23]

Hi all! This forum has been unbelievably helpful, but I couldn't seem to find an answer via searching, so apologies if I missed something!

We're in the SF Bay Area and qualify for the $1/W Equity Resiliency rebate. Our average monthly usage is around 335kWh (going back 3 years), with a peak daily usage of 23kWh (average is a little over 11kWh). No AC, no pool.

Most installers we've spoken to have taken our average usage and recommended a system right around 3kW with a single Powerwall. Given the likelihood of PG&E having extended PSPS events, we were considering getting two Powerwalls to handle the longer outages as well as to be able to do whole-home backup with the 60A.

The SGIP Equity Resiliency rebate requires 52 complete discharges of the incentivized kW over the course of the year, which I don't think would be too hard to do? The other component is charging two Powerwalls from a 3kW system would just take forever if most of the solar goes to powering the house.

With all that said, here's the questions:

1) Is there any scenario where having two Powerwalls tied to a small system like mine makes any sense? I've seen the "1 Powerwall for each 5-6kW" rule of thumb tossed around, so I'm thinking probably not, but a guy can hope!

2) PG&E's NEM interconnect rules require sizing a system to max 120% of past 12-month average use. Is there any wiggle room there if we wanted to add some panels in order to help charge two Powerwalls? There's a non-zero chance we get an EV at some point in the next few years.

Cheers everyone!

 

[holeydonut]

Hi Supralover - welcome to TMC!

The 52 "complete discharges" is a bit misleading. You just need the Powerwalls to push out energy that equals 52 cycles. So it's not like you have to go from 100% to 0% 52 times. It's more like you'll go from 90% to 70% 260 times.

With 2x Powerwalls, you'll need to push 1,374 kWh over a year (687 kWh per battery). I'm guessing your annual usage is around 4,000 kWh per year; so I think this is reasonable for you to be able to consume 1,374 kWh of battery power a year. This is especially true if you add a EV later that draws some of its charge from your stored Powerwall energy. The key for you would be to make sure you're consuming electricity when the sun isn't up; ensuring that you're discharging those batteries.

 

[jrweiss98020]

I'm in Seattle, not SF, but here's a few numbers to consider...

I have a 9 KW system and 2 Powerwalls. Max system production is ~65 KWh/day summer and 22 KWh/day winter. First year (less 9 days) production was 10.3 MWh, or 860 KWh/month. I use an average of 30 KWh/day. Based on these numbers, and if you get a 3 KW system (and SF sun is similar to Seattle), you would see a max of ~30 KWh/day summer, 7 winter, 285 KWh/month average. About 3.5 KW would give you break-even production over the course of a year.

In the summer I can drain my Powerwalls to 65% overnight and have them topped off well before noon the next day. In the winter, it takes several days to recover if I use the Powerwalls, so I put them in Backup Only mode.

I don't know what "52 complete discharges of the incentivized kW" means, but you should be able to recharge your 2 Powerwalls daily in the summer, and you would have several days of backup power available in the winter.

AFAIK, the 7.6 KW per Powerwall is a MAXIMUM recommended PV system, so the Powerwalls can accept any inrush. With the lower PV output, you just have to learn to manage your Powerwall discharge...

 

[MorrisonHiker]

As you mention, having 2 Powerwalls will allow you to back up the entire house. With only 1 Powerwall, you'd only be able to back up predetermined circuits and nothing over 30 Amps IIRC.

I don't think you can count charging of the Powerwalls in your 120% but your utility might have a rule allowing you to add more panels if buy an EV.

 

[jjrandorin]

As you mention, having 2 Powerwalls will allow you to back up the entire house. With only 1 Powerwall, you'd only be able to back up predetermined circuits and nothing over 30 Amps IIRC.

I don't think you can count charging of the Powerwalls in your 120% but your utility might have a rule allowing you to add more panels if buy an EV.

I believe most utilities in CA allow one to file a document with their install that says you anticipate using more power than your previous history. I filed one of those with my install in 2015, and remember reading someone else here in CA filing one this year. It basically says you anticipate an uptick in usage in the next 12 months, OR that you have not lived in your property for 12 months so the history is not accurate to your usage.

Purchasing an EV (for OP, for example) could easily double OPs energy usage, depending on how many miles driven.

 

[holeydonut]

I'm in Seattle, not SF, but here's a few numbers to consider...

I have a 9 KW system and 2 Powerwalls. Max system production is ~65 KWh/day summer and 22 KWh/day winter. First year (less 9 days) production was 10.3 MWh, or 860 KWh/month. I use an average of 30 KWh/day. Based on these numbers, and if you get a 3 KW system (and SF sun is similar to Seattle), you would see a max of ~30 KWh/day summer, 7 winter, 285 KWh/month average. About 3.5 KW would give you break-even production over the course of a year.

In the summer I can drain my Powerwalls to 65% overnight and have them topped off well before noon the next day. In the winter, it takes several days to recover if I use the Powerwalls, so I put them in Backup Only mode.

I don't know what "52 complete discharges of the incentivized kW" means, but you should be able to recharge your 2 Powerwalls daily in the summer, and you would have several days of backup power available in the winter.

AFAIK, the 7.6 KW per Powerwall is a MAXIMUM recommended PV system, so the Powerwalls can accept any inrush. With the lower PV output, you just have to learn to manage your Powerwall discharge...

In California, there is a Self Generation Incentive Program (SGIP) for battery storage. In this case SupraLover is eligible for $1 per battery kW worth of incentive since his home was impacted by power instability as our utilities tried to prevent mega forest fires. So that'd be about $13,000 from the California Public Utility Commission for each Powerwall.

But this program was not purely intended for people to just store and save energy; it requires the batteries to be "cycled". It sounds like SupraDude would still need to adhere to the small-scale program (1x or 2x Powerwalls) rules even though he's getting his Powerwalls due to grid instability/fire-risk.

So each Powerwall would need to complete the equivalent of 52 total cycles per year. This would be 867 kWh worth of energy being discharged by each battery each year. The large-scale program is even more strict and requires even more cycles to achieve the SGIP rebate cycling target.

 

[MorrisonHiker]

I believe most utilities in CA allow one to file a document with their install that says you anticipate using more power than your previous history. I filed one of those with my install in 2015, and remember reading someone else here in CA filing one this year. It basically says you anticipate an uptick in usage in the next 12 months, OR that you have not lived in your property for 12 months so the history is not accurate to your usage.

Purchasing an EV (for OP, for example) could easily double OPs energy usage, depending on how many miles driven.

That would be helpful! I don't think we have that option here in Colorado. When we put in solar in 2018, we were limited to 120% but could also show purchase of an EV and that allowed us another 3000 kWh production. We were actually able to put in 141% even though we knew it wouldn't be sufficient. After waiting a year, we were able to show our first solar system wasn't large enough and we were allowed to put in the additional 4 kW system. Normally, that should've been enough to cover close to 100% of our usage...but thanks to me WFH in 2020, we're now generating about 200% of our annual usage.

 

[miimura]

I have a 4.3kW solar system that was installed in 2012. I got two Powerwalls in 2018. It works great. I wish my solar was bigger to compensate for charging two EVs. That would reduce the amount I owe at the end of the year. The daily use of the Powerwalls to offset Peak usage is fine as it is.

 

[jrweiss98020]

In California, there is a Self Generation Incentive Program (SGIP) for battery storage. In this case SupraLover is eligible for $1 per battery kW worth of incentive since his home was impacted by power instability as our utilities tried to prevent mega forest fires. So that'd be about $13,000 from the California Public Utility Commission for each Powerwall.
. . .

So each Powerwall would need to complete the equivalent of 52 total cycles per year. This would be 867 kWh worth of energy being discharged by each battery each year.

How can the utility measure the cycling of the Powerwalls? Do you have to send the logs from the Tesla app? I just found I have drawn1965 KWh from mine in the last year (less 9 days).

That 867 KWh you cite is about 21% of the OP's total consumption. I think he would have to set a 65-75% Reserve level all the time to ensure he cycles enough power through them. Of course, he could monitor the draw through the summer, and reset to Backup Only when he hits the minimum, so he can have 100% reserve in the winter...

 

[jjrandorin]

How can the utility measure the cycling of the Powerwalls? Do you have to send the logs from the Tesla app? I just found I have drawn1965 KWh from mine in the last year (less 9 days).

That 867 KWh you cite is about 21% of the OP's total consumption. I think he would have to set a 65-75% Reserve level all the time to ensure he cycles enough power through them. Of course, he could monitor the draw through the summer, and reset to Backup Only when he hits the minimum, so he can have 100% reserve in the winter...

Tesla tracks it, I am fairly certain. I remember reading about it somewhere.. I thought there would be some sort of notice from tesla if you were not on track to cycle the batteries enough. I use self powered mode and go from 100% to roughly 30% every day on two powerwalls, so I am not concerned about meeting those guidelines.

I also havent gotten any sgip, and am doubtful I will get any, even though I installed in Jan of this year.

 

[holeydonut]

How can the utility measure the cycling of the Powerwalls? Do you have to send the logs from the Tesla app? I just found I have drawn1965 KWh from mine in the last year (less 9 days).

That 867 KWh you cite is about 21% of the OP's total consumption. I think he would have to set a 65-75% Reserve level all the time to ensure he cycles enough power through them. Of course, he could monitor the draw through the summer, and reset to Backup Only when he hits the minimum, so he can have 100% reserve in the winter...

I'm paraphrasing what @Vines told me, but it seems like there is no strict reporting around annual cycles to the CPUC or to your local utility. Rather, the utilities reserve the right to audit a homeowner if there is reason to believe they are not cycling. So that means a homeowner must still be able to track their cycling or else they'd have no evidence to produce if audited.

But on Tesla's website, it seems like their Powerwall app provides tracking and gives users the ability to guide cycling toward targets. I don't have Powerwalls yet, so I can't tell how easy this is to set up. I also can't tell if they let you pick the # of cycles. The small-scale program needs the equivalent of 52 cycles per year while the large-scale program needs 104 cycles.


Energy Incentives | Powerwall Support

Tracking the Annual Usage Requirement

To ensure you are tracking towards the 687 kWh goal, simply return to the Tesla app home screen, select Power Flow, and click on the Powerwall icon. From here you can choose a time period to track your usage.

On average, you will want to use 57 kWh/month. We anticipate all our customers will meet this minimum requirement provided they follow the recommendations above regarding Backup-Only and the backup reserve.

If your system is not on track to meeting the annual usage the SGIP program requires, certain settings of your system may be restricted; for example, limiting the share of the system’s capacity you can reserve for backup. This may occur 8 months into the year and on the anniversary of that date for the next four years of the program. This would be a temporary restriction, with full functionality restored once you have met the program’s cycling requirements.

 

[getakey]

In California, there is a Self Generation Incentive Program (SGIP) for battery storage. In this case SupraLover is eligible for $1 per battery kW worth of incentive since his home was impacted by power instability as our utilities tried to prevent mega forest fires. So that'd be about $13,000 from the California Public Utility Commission for each Powerwall.

But this program was not purely intended for people to just store and save energy; it requires the batteries to be "cycled". It sounds like SupraDude would still need to adhere to the small-scale program (1x or 2x Powerwalls) rules even though he's getting his Powerwalls due to grid instability/fire-risk.

So each Powerwall would need to complete the equivalent of 52 total cycles per year. This would be 867 kWh worth of energy being discharged by each battery each year. The large-scale program is even more strict and requires even more cycles to achieve the SGIP rebate cycling target.

you are referring to the Equity Resiliency SGIP. It is $1/wH and you have to be located in a high fire threat area or other criteria. There is another SGIP that is lower and has limited fund allocation per installer

Discover the Self-Generation Incentive Program for non-residential customers

 

[holeydonut]

you are referring to the Equity Resiliency SGIP. It is $1/wH and you have to be located in a high fire threat area or other criteria. There is another SGIP that is lower and has limited fund allocation per installer

Discover the Self-Generation Incentive Program for non-residential customers

Yes, OP mentioned they were doing Equity Resiliency. I was just pointing out that based on OP's commentary he would still have to do the 52 cycles per year even though his primary use is for Resiliency backup.

 

[getakey]

I don't see how anyone in Bay Area would qualify for Resiliency

 

[holeydonut]

I don't see how anyone in Bay Area would qualify for Resiliency

I guess it depends on how big you draw your "Bay Area" map haha. The Tier 2 threat areas on the ArcGIS map are plentiful. I guess homeowners in the Lamorinda area and the Oakland Hills area would be ideal candidates.

CPUC FireMap

 

[supralover23]

I don't see how anyone in Bay Area would qualify for Resiliency

I'm up in the East Bay hills in a Tier II zone, plus we had more than 2 PSPS events last year. We also are on Medical Baseline for a CPAP machine.

I really appreciate all the replies! Looks like we're going to move forward with the two Powerwalls, then add some panels down the road if we have issues keeping them charged.

 

[JPP]

I don't see how anyone in Bay Area would qualify for Resiliency

Huh? Tier 2 and 3 fire zones--check. PSPS-check. Medical need or economic need or well pump--check.

FWIW the plan opened up early March and AFAIK they have run out of the first chunk of $$.

 

[supralover23]

Huh? Tier 2 and 3 fire zones--check. PSPS-check. Medical need or economic need or well pump--check.

FWIW the plan opened up early March and AFAIK they have run out of the first chunk of $$.

Unless I'm reading this wrong, there's still $117 million available for equity resiliency.

SGIP |

 

[getakey]

I'm up in the East Bay hills in a Tier II zone, plus we had more than 2 PSPS events last year. We also are on Medical Baseline for a CPAP machine.

I really appreciate all the replies! Looks like we're going to move forward with the two Powerwalls, then add some panels down the road if we have issues keeping them charged.

Got it. Makes sense now

 

[charlesj]

To the OP.
I installed a small system in 2012, 17 panels with 215W inverters each . 235w panels. Inverters can handle up to 226W before clipping.
I had 2 PW installed on May 21. Charges and handles house well.