The cost of the system (assuming it is owned and not leased) is "sunk"; the contributory value of the system (if it exists, and there may be one) is dependent on anticipated net-savings of utility bills. There may be an additional "I'm Green" appeal in certain markets.
So the relationship between cost and value is based on the future benefits (savings) as judged by the market and not based on some percentage of the actual cost. Don't rely upon someone offering a set-percentage (Note that Mark Hurlock's 30% is based on a subject-specific
market analysis/extraction, and not some arbitrary number).
I recommend you use the search function on this topic; there is a lot of good information contained in prior threads.
Also, I will caution you (and you may think I'm being overly cautious, which is fine
).
Presuming that your license level listed on this forum is correct, the value analysis of the system you are talking about could be considered "complex". By definition, a license-level appraiser cannot complete complex assignments (for FRT work, in California) without supervision and cannot become competent (even though USPAP allows it, the licensing provisions do not): complex assignments for a licensed-level appraiser would need to be done with a certified appraiser.
If you complete the assignment and the borrower is unhappy regarding the outcome (rightly or wrongly), and turns the report into the state, then the state will make a determination
if the analysis (and the assignment) is complex or not. Even if you do everything letter-perfect (and I'm fine with assuming you will
) and the results are credible, the state is faced with having to decide if the assignment is complex. Obviously, if the state finds errors with the report, it would be easier to conclude the assignment was complex from the get-go.
So, take my advice for what you think it is worth, and be careful on how you proceed.
Good luck!