See, this is why appraisers are often reluctant to talk to homeowners about atypical appraisal situations. We can explain how we do things but if that conflicts with their world view or the results don't match whatever their sales person and the eco-advocates are saying then most homeowners invariably retain their assumptions that it just can't be true.
You may be 110% correct and this appraiser may indeed have undervalued your home. Or, you may be incorrect and the appraiser may have done the right thing. You apparently have no market data that supports your opinion, so that puts you at the distinct disadvantage in a discussion about how your local market operates with respect to solar installs.
The difference between the appraiser's opinion and your opinion is that you have a desired result and the appraiser doesn't. Not only does the appraiser not care what you want but he is required to *certify* that he/she doesn't care what either you or their own client wants. The appraiser isn't on your team and they're not supposed to be on the lender's team, either.
That means the appraiser has no incentive to lie about their conclusion or to take sides. That conclusion may be in error, but if so that would be the result of making the assumption (which is what you have done) instead of actually looking for the data and working the analysis.
And before you go just dismissing their judgement and experience out of hand you might want to take a moment and think about what the judgement and experience consists of.
Our profession has added very few new fully licensed appraisers over the last 8 years so that means this appraiser most likely has at least 10 years of full time appraisal experience under their belt. That's because it takes a minimum of 2 years just to accrue enough acceptable experience to even qualify for the full license. So that's at least 10 years of analyzing - specifically for value - at least 100 local sales transactions a week and quite possibly a lot more than that. That's 10 years of completing and submitting appraisal reports that are accepted by various lenders for their use in making these decisions. Including their own reviews that are being performed by other appraisers and underwriters, each of whom have their own professional experience.
Based on what you've told us one of those reviewers apparently told you that they agreed with this analysis and conclusion, most likely within the context of what they've seen from a number of other appraisers in your region.
These are the people you're arguing with, and this is the level of information that they have accumulated in your market area which you have not accumulated. In counterpoint you've got an AI position paper that points to the use of an Income Approach to value this amenity; but what they didn't make as clearly as they should have was that this mode of analysis can only work to the extent that someone either arbitrarily *assumes* the direct relationship between savings and contributory value or else develops that opinion based on relevant data that actually demonstrates that relationship.
Just so you know, two properties can have the same income but result in different values in the market if they have different uses. So saying a $2500/yr reduction in net income can indicate to $25,000 in additional value to one property (say, a 10-unit apartment) doesn't necessarily mean it will have the same effect in a completely different property type (single family residence). That's not an opinion, it's a fact; and the AI's paper really should have made more prominent mention of that.