Interesting. I would have thought that you do it by (expected) lifetime of the panels. I haven't had my system for a full year yet so I don't have complete numbers, but it seems to be anywhere from .5 (winter) to 1.1 MWh per month.

Assuming I end up generating 8 MWh per year, I'll use that number along with an expected lifetime of, say, 20 years. So: lifetime production of 160 MWh, and my system (after incentives) was something around $18K.

$18,000 / 160,000 kWh = $0.11 / kWh.

If my system lasts 25 years, that goes down to 9 cents per kWh.

(note: I think I have a 10 year loan but will likely pay it off way earlier than that, but it wouldn't factor into this calculation other than interest)