I have done these in the past with an income approach method. First, you have to get historical buyback rates from your utility provider. Then, you must figure the life expectancy of each of the units...I have found the above post is correct for the panels at 15 years, but the transformer to convert the DC power to feed back into the grid had only a 5-7 year life, and the reserve for replacement of this item was very high and took a bunch of the potential return to the owner away. Once I figured the potential back to the owner, just use a GRM for the property type to come back to an adjustment. You can also do a depreciation cost method, however, I think the income method would be better support as you are talking about a reduction (or elimination) of a monthly bill for typical SFR users...that is what is motivating them.
Also, dont forget about maintenance of the panels, as we get snow, and maybe (if it is in an agricultural area) the owner would have to get up to clean off the panels, so that must be accounted for as well...
Good luck, the process and verification took me an extra day alone...I hope you are being paid by the hour, or else you probably will lose $$ on this one...