If I exclude my powerwalls, my 8.16 KW system was $12000 after the tax deduction. With the powerwalls it's saving me about $4500 / year. Without the powerwalls, it's saving me about $3300 / year or $66K over 20 years.
That's a bit less than 4 years to pay it off. Is my example typical? If so, then your example numbers might not reflect the scale of ROI on solar investments today.
Using today's dollars: if we very conservatively assume your system will be worth $0 after 20 years (which it likely won't but certainly won't be worth $12k) then your $12k investment would net you $66k over that time period or an annualized rate of return (ARR) of ([$66k - $12k] / $12k / 20) = 22.5% ARR. That's very good, in my area where electricity is very inexpensive and tax incentives are generous the ARR is typically ~10%.