>> will add between $8,000 and $12,000 to a home's cost
To judge this, we'd need to know more than just that one factoid.
Specifically, what would adding solar panels do to the ongoing monthly utility costs for that home? Reduce them, one would hope ...
We'd need to compare the total monthly cost of [Mortgage plus Utilities without solar] to the monthly cost of [Mortgage plus Utilities with solar].
If the net monthly costs were lower with solar, the upfront cost would be easy to defend.
A caveat here would be that lending institutions might need to tweak their formulas so they take reduced utilities into consideration when calculating someone's eligibility for a mortgage. They'd need to look at more than just the monthly mortgage payment on its own.
The same could be said about other strategies for reducing the energy footprint of a home (extra insulation, passive solar design, etc). They'd usually add to the upfront costs, but would reduce ongoing costs. Lenders need to calculate accordingly.