I finally figured out (I think) how to calculate our solar savings with reasonable accuracy. The figures below go back only nine months, because that was when PG&E started giving us enough information on our bills to be able to calculate our savings properly.

Even so, the results are only approximate, because I didn’t feel like putting in the work required to account for every wrinkle in PG&E’s byzantine pricing structure, but I think they’re close enough.

Over the nine bills (January-September), we paid PG&E $628.61. If we had had to pay PG&E for every one of the 12,700 kilowatt-hours we used, we would have paid about $3,600, so we saved $3,000.

Our average price per kilowatt-hour would have been $0.2835. (Electricity is expensive in California.) Instead we paid PG&E an average of $0.049 per kilowatt-hour.

If this pattern holds, and we save $4,000 a year, we’ll have recouped our investment in five years. That will be April 2019.