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Moving to new house.. solar system sizing

PG&E (San Jose Clean Energy) customer. I will be moving into a new home in the next few months and want to install solar as soon as possible to start saving money on my energy costs. I've settled on Tesla solar panels and I currently already drive a Model 3 with 2 years of free supercharging.

In the next two years, I think 4.08kW system should cover most of my electricity needs, but should I install 8.16kW to future proof? I estimate that my energy bill will be approx $100-$130/month during the next two years. Based on Tesla savings estimation, 4.08kw will cover $92 of the estimated ($100-$130/month) energy bill each month. I plan to replace the gas wat/er heater with a hybrid heater and also replace the gas cooktop with induction. I will likely need to start charging at home once my free supercharging ends and my partner will likely get an EV 5-7 years from now. What would you do in my situation? 4.08kW will cover the majority of my energy needs while 8.16kW system will definitely be oversized but in 3-5 years my energy needs to increase due to two EVs. Not sure whether it makes sense financially to install 4.08kW and install add-on panels later on, or just go big at the beginning with 8.16kW. I'd love to get a powerwall but from ROI perspective it doesn't seem to save any money and more for backup power.

As I understand it, there is a minimum $10/month bill with PG&E even if I cover all my energy needs from solar. But San Jose Clean Energy also provide 4.19cents/kW for any excess (

Will the $ generated from excess energy generated be sent to me via a check or does it stay as a credit on the power bill and rolls over into future years until it can be consumed? If I receive a check, then technically the excess power I generate can pay towards the $120 annual minimum bill?

Should I submit an order for solar as soon as I move in, or wait a few months to see my energy usage trends?


  • Here is a suggested guideline to estimate actual production (based on 10+ years data in NorCal - Napa Valley). Each kW of solar size should produce in excess of 100 kWHrs per month on average. My system produces about 135 kWHrs per month per kW of size. Your actual production will depend heavily on orientation and shading...mine is primarily 240 degress (WSW) and has zero shading. Keep in mind that is ANNUALIZED and winter production is typically under half the average with summer going the other direction.
    Look at your current bill not in terms of $, but in terms of kWHrs used per month. I have gas water heater and gas heating - all else is electric. I also have AC and drive a model 3, with near 100% charge at home (previously had Bolt and Leaf dating back to 2013).
    10 kW system on my 2,000 sf home and I produce about 7% above my annual consumption as a cushion and to have useful winter production.
    Extrapolate your current energy usage in terms of square feet from your current home to your new home with the same appliance mix.

    1) I predict the water heater will hammer your electricity consumption due to the resistance backup (unless your hot water consumption is after the sun has had some impact on your solar).
    2) The car (if you eventually charge at home) will suck up enormous amounts of electricity.

    4 kW at the time of my initial install in 2007 was an enormous system - it is now pretty minimal. Avoid the pain (and expense) of additional installs and put up a system that is future proofed. Don't plan on useful money in your pocket for excess production - the utilities know how to create rate structures that are not to your advantage - it's what their shareholders expect and they do it well. BTW - they do send you a check once per year for your overproduction.

    Reconsider PowerWalls. When the grid goes down - your solar system is just a silicon shade without a battery. I predict that outages are a simple fact of life - look at our local history and Texas in the past few years as examples. Climate change and reduced hydropower is going to make rationing a fact of life at some point. Thats' a pretty grim assessment, but not unrealistic on the whole.

    Bottom line - think in terms of kWHr usage and not $ when you size your system.

    You will get lots of advice from this forum - nearly all of it will come from people who have experience to back it up.
  • Also understand that utilities want to consider historical energy usage patterns, and will try to use that to determine allowable average yearly production. They DO NOT want to be in a position to pay you year after year. The typical limit is 110% of historical usage.

    That's going to be a problem if you didn't live at that address in previous years.

    I was faced with something similar. I owned a rental house with tenants for fifteen years. The tenants moved out, we did a massive (to the studs) remodel, and installed solar PV as a part of the redo. I did my own calculations, knowing our historical usage at our previous residence, and added in the two EVs we had recently purchased (considering historical miles per year driven). I told the Solar City Concierge that my array was going to be 5kW. SC went along with it, but SCE (my utility) balked, and denied PTO.

    I provided my calculations to the Concierge, and they negotiated with SCE. I got PTO a week later, with one caveat: I had to sign a letter saying that I would not overproduce, and that if I did, there would be penalties. I don't know what the penalties would be, because my offset has been 85-90% since.

    I post this as a cautionary tale, as one can never know what their utility will do with a system that (in their minds) is too big. It never hurts to talk to your utility (in advance) to see where they are in all of this.
  • I recommend you get the larger system.

    I am with PG&E and SJCE and got PTO last month for a system that will overproduce by at about 30%. Tesla just put on the form - planning to increase usage by 4000 kwh.
    I was in a similar situation as OP trying to estimate what I need, I even changed my order to add PWs then changed it back.

    How often do you get outages? Check you outage circuit if you are 50 (no outages), then do not order PWs as they will not pay off based on time shifting.

    Then pretty much you have only choice between 4.08 and 8.16 kw systems. I chose the 8.16 to overproduce since because of TOU rates, I'd be getting low credit on the daytime production while charged expensive rates at night. But my bill was $200/mo for about 9000 kwh a year so that made sense.

    If your total consumption is about 5600 kwh a year, you would have that covered with 4.08 kwh system. However if you start charging at home 12,000 miles are 3,000 kwh, so it would certainly make sense to get the larger system. The heat pump water heater uses much less electricity and I do not recommend getting one since they are noisy. The induction cooktop uses even less.

    You can wait a few months, could make it easier to do the paperwork for the PTO.

    referral link:
  • An induction cooktop, while very efficient in the long run, can draw more instantaneous power alone than what a single Powerwall can provide. My cooktop (a Thermadore top-of-the-line...we LOVE it!) can draw up to 6kW under "Boost" for several minutes. A single Powerwall is only good for 5kW (7kW momentarily). We have two Powerwalls, for a total of 10kW of instantaneous power capability.
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