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)
That would certainly make the most sense to me, and maybe even based on the warranty period, with accounting for replacing failing or underperforming panels after that.
That way, the statement, which sounds too good to be true, "and then the cost drops to zero" doesn't enter into the conversation. Sure, the monthly cashflow cost may be zero for a while, but that might just be accounting trickery.