No offense to appraisers in general, but the very few that I have had to deal with couldn't manage to value a $2 bill in my estimation. One literally photographed the driveway of my property and three others in the county (not even this zip code) as a basis for an appraisal value. The other appraisals may have been more detailed, but were grounded in "comparable" homes that weren't actually comparable, other than, yes, they were houses (different age, design, bedroom #s, areas for both home and lot...) Regardless, what we happened to value in this particular parcel had to do with the classic phrase "location, location, location", which turns up on nobody's appraisal. My point being that properties change hands for sometimes the quirkiest of reasons. (e.g. Next door to mom, no backyard neighbors, has a tree swing, on a corner to show off, once belonged to _____…)
A nearby local home was designed by a deceased, internationally renowned architect, and one of a small handful in the US. That sale will certainly bump the county home sales price averages, but it is my belief that that particular sale has no relevance to virtually any other property transaction in the area.
Perhaps in a largely identical subdivision, you could run the numbers and work out what solar might be worth, but I think apples to apples home comparisons are tough for all of the idiosyncrasies in home buying. I overheard folks not bidding, or bidding on a home because of the colors, inside or outside. Talk about a cheap fix! I do think that the point of the potential added value of solar with respect to the magnitude of the home's price has merit. A potential 1% added value vs a 15% added value makes a difference in how easy it is to identify solar as part of the buying decision. (And "a home with solar" is a broad brush; there are folks here with 4kW systems and 40kW+systems.)
At the end of the day, when the house sells, you know the value of that house that day to those two parties; one was happy to sell, and one was happy to buy at that valuation.
All the best,
BG