Solar Panel Cost Summary from Tesla Roadster

[Owner]

From my blog, I thought I would crosspost here and start a discussion.

I have had my solar panels now for 6 years, 3 1/2 years of Tesla Roadster driving and charging. I number crunched my data exhaustively and here are my conclusions.

1. Charging the car took 10,276 kWh to drive 29,000 miles. This correlates to a 2.82 miles / kWh. The listed rating is 3.3 miles / kWh.
2. The system will pay for itself in 5 more years. The first 2 1/2 years without the car only put a few hundred dollars back into the solar because of low usage. I had planned the system while waiting for the Tesla Roadster! So the total number of years with an electric car is about 8 years.
3. If I didn’t have solar, my outlay per year for the car at about 7,500 miles per year would be right around $2,000 to charge the car with the standard E-1 rates.
4. I am currently on E-6 rates with PG&E. I definitely don’t want the second E-9b meter and the large upfront cost. I also am not very interested in the E-9a time of use periods that are much more restrictive than the E-6 times of use. I think the E-9a rates may make sense for someone whose car charging greatly exceeds their home usage. But I have not done an extensive calculation to compare the two rates.

My solar system is 3,825 Watts and has an annual estimated production of 6,885 kwH. Total net cost of the system after rebates but including the permit and some solar panel cleaning was $17,750.
There are many caveats to these conclusions. I expect some feedback on my assumptions. This post was not an easy task, and I assume I have made some errors. I found it quite difficult to create completely accurate data and these conclusions are only good estimates because of the following:

1. The car does not report any useful data such as kWh used for a reasonably long period like a month or a year.
2. PG&E data is only the net energy you used. Unfortunately it does not give you a breakdown of energy you generated. Nor do they have any particularly helpful tools on their website.
3. My inverter does report the amount it generated on a daily basis, but I don’t have the time or patience to make a record of this.

Assumptions I made:

1. I charged the Roadster after 10pm in non-peak hours throughout the period with less than a handful of exceptions.
2. I estimated the Roadster charge to be the difference during these periods post-Roadster and pre-Roadster.
3. I used someone else’s estimate for monthly variation of solar generation to estimate the amount of power I generated per month.
4. I am only using the current PG&E rates to calculate the amount I would have paid without solar, not estimating it more accurately with their frequent rate changes.
5. I estimated the amount I generated based upon the solar panel installer’s estimate and the numbers do seem to correlate with what I see when I look at the inverter’s screen readout.
6. I had to do some number estimation to put in the monthly baseline calculations.
7. I am not considering the time value of money.