> I would have thought that you do it by (expected) lifetime of the panels.

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.