You are close. I understand the roof mount inhabitions as I would rather a ground based system.
I sized my system to just under 8kw/hr. No batterries, grid tied.
Buying wholesale and installing myself gets me near your $12k. Factor in the 30% Federal tax credit and I'm down to under $9k.
I have Eversource and the way it works is you build credit for the energy produced, minus what you use.
If I average 4hrs of peak sun per day (just peak sun), thats 960kw of sun per month. Minus my yearly average of 700kw/month gives me a monthly credit of 260kw of electricity. None of this calculates the off-peak hours of production. Remember, panels still produce electricity, even on cloudy days (this can be around 60-70% efficiency).
So, by all my calculating, I've paid off my system in under 8yrs. If I dont plan to sell, until after this time, who cares if a buyer doesn't want it? I'm not locked into a contract and it's paid for itself. Tell them you'll rip it down once the deal is closed.
I understand that not everyone has yard space, they may need a new roof and have to hire a contractor. So, their startup costs and ROI will differ greatly from my situation. I have no problem whacking down some trees and hiding an array on my property. Plus, you can get an 8kw, lp, kohler generator for about the same price as a decent battery backup system (which would need maintaining and replacing).
The real problem is SolarWorld. Because they're declaring bankruptcy, they're bitching and whining about imported solar panels being cheaper. They're pleading with lawmakers to tax these panels, which would price solar out for a lot of people. The majority of premium solar panels are made outside of the US. If anyone is thinking about soar, pay attention because you may have to act soon.