My rough calcs, recently updated:
10 year payback
Cost of capital 7%/yr
Utility electricity $/kwH inflation 7%/yr
(Edit: Currently avg rate in Portland is appx $0.10/kwH. Utility is requesting +9% increase this year, after a +6.8% increase last year. I'm assuming +7%/yr inflation in utility electricity rate. Every +% increase in assumed inflation rate adds appx $500 to economically justifiable cost of a home system for me. But, I'm already assuming a significant inflation rate.)
Initially cut my electricity consumption (in kwH) 30%, then hold kwH constant
I can justify an upfront cost of appx $9K for the system, or $15K including current tax incentive ($6K tax credit over 4 years)
(Edit: I'm disregarding current federal tax credit since
AFAIK it expires this year. Most likely gets renewed, I'd think, but not going to count it until I see that law passed/signed.)
Assuming no change to tax incentive, I need installed $/watt for the system to decline appx 50% before the project becomes economic.
I haven't dug into it to figure out how long that 50% might take.