Figuring out optimal PV system size (+PW?) with PG&E NEM and EV-A

[joemo]

Hello, I am planning on installing a solar system on my house in the Bay Area in California, but I'm unsure my calculations on optimal system size are correct when factoring in NEM on the EV-A rate plan with PG&E. My communication with Tesla has been spotty, so I'm looking for some independent advice.

TLDR, Considering installing ~4.41kW system +1 PW for 8200kWh annual usage. Is this about right?

Here are some of my calculations based on past usage and best guess of future usage. (some figures rounded)

Per year I expect to use approximately 8200kWh. Of this amount about 52% is off-peak (.13/hWh), 29% is partial peak (.26/hWh), and 19% peak (.48/hWh). Off-peak includes things like electric laundry, dishwasher, and charging my Model 3. We don't have AC, and my furnace and hot water are gas.

Tesla estimates I would produce 1382kWh per kW solar installed. Of this amount, about 61% would be produced during partial peak, 38% during peak, and less than 1% during off-peak.

Because of NEM with PG&E, and because I'm able to use a majority of my power during partial peak and off-peak rates, I don't need to produce as much as I use. But figuring out the sweet spot has been a challenge. My estimate is that a 4.095kW system (13 x 315W panels-- I slightly prefer the all black look) or 3.96kW (12 x 330W panels) would be about breakeven on usage cost and production value. I would like to have a buffer on top of this to account for more usage in the future, but that is just hypothetical at this point since I have no plans to increase usage. I'm thinking 4.41kW system (14 x 315W panels).

I'm waffling on the Powerwall but I am considering it still. Economically it doesn't make sense, since I could add a lot more panels for the cost of the PW. I think the most attractive thing to me is not having to worry as much about shifting usage to off-peak times. Future rate increases (PG&E is in bankruptcy of course) or changes to the EV-A rate plan could also impact my calculations. The extra tax write off and the battery backup are bonuses but not a huge factor for me. My end goal would be to have power bill as close to $0 as possible.

Thank you very much for reading, and please let me know if you have any thoughts!

 

[Silicon Desert]

If you haven't done it already, get at least 2 other quotes from other companies.
For a couple reasons you mentioned, I am not a fan of the Powerwall, but that is just me when I did the analysis for one of my systems. It makes sense to have it for some people, just not my Northern California home. Just keep in mind that while it might be nice to have a 0 power bill, it is sometimes not economically the best decision. Example, in my Northern Cal home, as I add more modules, the return on investment becomes longer because that energy is offsetting cost in a lower rate tier, especially if the orientation isn't perfect for the sun.

 

[Kermit]

That production doesn't sound high enough unless you have poor sun exposure for your panels. If you panels are tilted south at an angle = to latitude (37deg for SF Bay Area) you should get 5.0 to 5.5 kWH per day per installed kW. Solar Insolation Chart, Solar Insolation Data | Average USA Sunlight | Solar Energy Radiation Map
So for a year, that's 2000 kWh per kW
Also, about 1-1/2 years ago my city switched to Community Choice Energy (Silicon Valley Clean Energy) and just this week to my surprise, they cut a check for the excess $ production over the first annual cycle. PGE never did that. So I think my system is a little oversized on a dollar basis, but not on an energy basis.

 

[SoundDaTrumpet]

A solar design white paper was published Aurora Solar. It did conclude that current NEM solar design departs from building a system that covers 95% of annual energy use now to 110%. You may be better off relying on the solar size they are providing you. Apparently I learn that SunWorks, my installer, uses their modeling software. Not sure how Tesla does their sizing. Ask what they are using in their model, or find another installer.

EV-A is changing. An expected plan phases in July 2019. Because you haven’t reach PTO on any solar or energy storage system (ESS), your EV-A rate schedule will shift to 4-9PM peak and 12AM-3PM off-peak. A solar or ESS will grandfather those in. This will vastly affect your projections.

PG&E will always be revenue neutral with rate changes and time of use restructuring. The new EV rate plan is designed for ESS ONLY in my opinion. I stress that. All rate (E-1, ETOU-A, ETOU-B) plans according to PG&E rate comparison tool are exactly the same annual bill today. Current EV-A is half the cost. The rate change to the new EV rate schedule using my fancy spreadsheet brings the annual cost equal to all other rates. There’s no advantage for one rate plan over the other coming July. With this in mind, I feel rate arbitrage with solar alone is a lost cause.

The white paper pointed out maximum production and south facing was appropriate for past NEM policies. Today, it’s maximizing afternoon production.

 

[miimura]

A solar design white paper was published Aurora Solar. It did conclude that current NEM solar design departs from building a system that covers 95% of annual energy use now to 110%. You may be better off relying on the solar size they are providing you. Apparently I learn that SunWorks, my installer, uses their modeling software. Not sure how Tesla does their sizing. Ask what they are using in their model, or find another installer.

EV-A is changing. An expected plan phases in July 2019. Because you haven’t reach PTO on any solar or energy storage system (ESS), your EV-A rate schedule will shift to 4-9PM peak and 12AM-3PM off-peak. A solar or ESS will grandfather those in. This will vastly affect your projections.

PG&E will always be revenue neutral with rate changes and time of use restructuring. The new EV rate plan is designed for ESS ONLY in my opinion. I stress that. All rate (E-1, ETOU-A, ETOU-B) plans according to PG&E rate comparison tool are exactly the same annual bill today. Current EV-A is half the cost. The rate change to the new EV rate schedule using my fancy spreadsheet brings the annual cost equal to all other rates. There’s no advantage for one rate plan over the other coming July. With this in mind, I feel rate arbitrage with solar alone is a lost cause.

The white paper pointed out maximum production and south facing was appropriate for past NEM policies. Today, it’s maximizing afternoon production.

Do you have a reference for where it says that only solar and ESS will be grandfathered. In all other cases I can recall, anyone on a rate plan that was closed was allowed to stay on during the grandfather period. It would be strange to force the change on EV owners without solar. It may be to their benefit if they have no solar or battery, but discriminating against a particular group of customers would be unusual.

 

[miimura]

If you are charging an EV at home, your system seems small to me. I have paid about $500/year per car to PG&E with my 4.3kW solar system. I think the system would only offset my household usage, not my EV charging at all. I have some winter shading due to a very tall tree that is a couple houses away from me and I'm up against the western hills so my afternoon generation drops off fast.

The changes to the EV rate schedule will also devalue most of your generation down to the Off-Peak rate with the change to Off-Peak hours going to midnight to 3pm. That will be a huge loss because you will be charging your EV at 1:1 kWh instead of 2:1 for your Part-Peak generation or 3:1 for your Peak generation. I will definitely have to add more solar before I'm forced onto the new schedule.

 

[Ampster]

I agree adding more solar for EV charging is a good idea. Also, a factor for me in considering a Powerwall is the frequency of power outages. I am in PG&E, drive two EVs but am also in an area impacted by fires. What that means is PG&E may take down my section of the grid when there are high winds or other events that may cause their wires to start fires.

 

[SoundDaTrumpet]

Do you have a reference for where it says that only solar and ESS will be grandfathered. In all other cases I can recall, anyone on a rate plan that was closed was allowed to stay on during the grandfather period. It would be strange to force the change on EV owners without solar. It may be to their benefit if they have no solar or battery, but discriminating against a particular group of customers would be unusual.

@miimura I am afraid it is an assumption taken on my part. From reading the white paper, I benefited from the notion that both solar systems and ESS are strongly dependent on the existing rate structures and NEM policies in place. Meaning a PG&E customer makes a significant investment (10-25 years) and to not upset homeowners... hence the grandfathering. I believe EV owners are not viewed like so.