Pre Solar Inquiry


I am getting ready to go solar. I installed sense and was wondering how to size a solar system with the readings from sense. Not to familiar with all the electric measurements. I’ve had estimates between 12-15 KW systems. Installed Sense 4 days ago so it hasn’t identified much. Solar adapter is waiting for install. My biggest concerns are 2 sump pumps, heavily used, and a Tesla Model 3 w/48AMP home charger. I want to get the right sized system.


Look at your utility bills from the last year to determine your annual consumption. Find out what your electric utility pays you for excess power exported to them at annual “True Up” time. If the payback from the utility $/kWh are less than what it cost you to produce that power, size your system for 98% of your needs. If the payback is higher than what it costs you to produce, put in the largest system your utility and your wallet will allow. In my case, I have a 15kWh system because my utility pays out at “True Up” time $0.129 per kWh. I am on track to get about 1100 back from the utility next December and pay them ZERO per month. My payback to 100% is 9 years and 8 months. Do your homework and get all the details. Good luck!


Thanks dwleckie, The EV has only been with us 8 months and the kids have recently moved out so last year or two will be different. Things always changing. I was hoping to get 1-3 months Sense data and calculate for yearly consumption. Sense has not discovered my EV yet! My utility net metering is 1 for 1 KWh and they don’t cut checks anymore.


The metrics are fascinating aren’t they?

I suppose like many EV owners your solar sizing, you would think, is going to depend upon how much you drive!

Tesla 3 runs at 26kWh/100miles

If you’re in a good solar zone, let’s say Vegas, a “10kW” premium system is going to average around 1325kWh/month according to our government. In December (minimum solar supply) you’re looking at 807kWh. Ignoring the rest of the household requirements, worst case month (December) will get you 807/26 = 3100 miles!

Did your estimators give you any metrics? As @dwleckie points out checks back from the Utility could give you incentive and influence that calculus but it would seem short-sighted to rely on that income (in general, I know it doesn’t apply in your case). Introducing a battery (PowerWall or whatever) into the mix and factoring in your Tesla driving patterns (and its battery) also starts to get very interesting when you’re installing a larger array. The rough “old” metric of getting as big an array as you can afford because you can get checks on the excess supply would still seem to apply to a certain extent: if you can get sufficient batteries to buffer the excess!

Is there a nicely integrated calculator for all those factors?

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i’m not sure a three month estimate will be accurate depending on your weather extremes, heating and cooling costs can vary significantly over a year.

don’t assume anything you ship down the line will be paid to you at the rate you pay them. I pay retail, 11 cents, for anything I use over what I produce bur they only pay me 8 cents for what I over produce.

I rounded my estimated annual usage up in deciding the size of my solar array given that there is an expected degradation over time and I’m currently getting a nice little bonus from the electric coop at the end of the year. I do, however, pay a $25 monthly minimum “meter” fee just to be on the grid so that’s a question you need to ask and include in your budge.


I built a custom calculator (long after I installed solar and Sense) to look at tradeoffs between solar, car charging (Model S, Model 3), plus various net metering rate schedules from my utility. My opinion - a lot depends on things you haven’t mentioned.

  • available net metering rate schedules - tiered, time of use (ToU)
  • costs and payments associated with those schedules - do you get wholesale or retail when selling back to your utility ?
  • how much flexibility do you have with your energy loads ? - i.e. charging car during the day vs. late at night.

For example, if you are billed by ToU, but only paid wholesale above that, then it might make the most sense to size your system to your needs during the most expensive peak ToU period, to maximize return.


Good points @kevin1.

Did your custom calc include added PV vs PW(s) (PowerWall/s)?

The flexibility issue you point out I think is key and also heavily tied (IMHO) to a choice of added house (battery) capacity vs added generating capacity.


I included PV, but not PowerWalls into my calc for a couple of reasons:

  • Adding storage to the mix adds even more complexity
  • I don’t yet need to add that dimension to my situation since my solar was already sized by Tesla/SolarCity to provide less than my on-peak and partial-peak usage. That my current net usage is computed on monthly basis means that any instantaneous power delivery over my usage is offset against my monthly total usage for each ToU, which is kind of like having a battery.

I guess there is one other driving facto in looking at solar - what are you trying to achieve ? Lowest cost or highest renewable content ? That adds more variables into the equation - I have the choice of buying whatever mix PG&E chooses to provide, ECO50 (50% renewables guaranteed) or ECO100 (100% renewables guaranteed). Or the choice to mix in more solar…


I’m intrigued by the calculus not only from a personal standpoint.

There’s a literal give & take: on grid / off grid.
There are global inefficiencies with inverting back to the grid, even if the energy is used somewhat “locally”.
There are obvious efficiency gains when consolidating resources in Utility-scale arrays that work against fully localizing generation.
That said, if you are in the Arctic you live in igloo.
Question is: What’s the energy equivalent of an igloo?
It seems like starting with matching your local generation & storage with usage is the ideal. You use what you can generate and no more. That would be ECO100 by default without getting into the mess of ToU and Utility dependency.
You’re right of course that adding storage adds complexity but my gut says if you are in good sun then the place to start the calculus is with the goal of 100% self-generation+storage.
Then again, batteries are expensive!

BTW, there is an argument to establish a system and then limit yourself to it’s capacity … we’re all in a lifeboat with limited crackers and nobody looks particularly tasty.

Sense kind of hints you in that direction already: like Screen Time for Watts!

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