Sales with supporting data are ideal, but if you don't have sales, then how would you value this - if not from DCF or costs? I recall reading a solar valuation framework that advocated using a 5% rate for the solar panel system, which doesn't seem appropriate unless we are in an ultra low-rate location/ market. Based on the R+G = Y framework, where R is consistent with the overall rate would be either consistent with that of the entire property or higher - due to being an improvement - the discount rate would thus be calibrated to the market. A response to this would justifiably be that this discount rate is market-derived for the property itself, rather than the market reaction to solar panels, and that point is valid. But, we are working with limited valuation data, and again, I ask what is a better approach.
Those statements are not incompatible, per above.