- 6.4 kW (16x400W panels), microinverter-based (little to no maintenance expected in 25 years)
- estimated to produce 8,500 kWh in year 1
- the panels are warrantied to produce 86% in year 25. Let's use 80% as a more conservative estimate.
- up front cost: $16,500 after the federal tax incentive
- alternative: paying PG&E $.25/kWh on average (conservative estimate)
To a first approximation, the system would save $2,125 in the first year, or almost 13% of the initial investment. In year 25, we'd expect 6,800 kWh. At the same energy price, it'd be saving 10% of the initial investment. (This doesn't take into account inflation, changes in energy prices, and I'm sure other things. This is all very different if your local utility doesn't offer net metering, too.)
The system breaks even in year 8, similar to what others in this thread have reported. But you can also look at it as a pretty low-risk investment returning 10-13% per year for 25 years. That sounds pretty good to me.