People generally are not willing to pay any more for a house with a generator
That might be a local situation. Our region has been blessed with 2 ice storms, floods, tornados, etc. for the past 3 years. Many of us have suffered power outages of from 3 - 20 or more days in those periods. The hospital in Joplin that was not destroyed ran on back up generators for over one week.... wouldn't you think that it was a functional inadequacy if a hospital did not have power back up? I certainly wouldn't want to be in the middle of an operation....
EO? A whole house generator is "On" the premises therefore is a functional, not an external issue. All the poultry farms in our area required them and they are automated to autorun if the power is lost and will run 5 min. after the power comes back on to make sure it lasts. They also "self-exercise" - every week the engine autostarts and runs for 20 minutes or so.
A "whole house" generator is generally an auto run system now. For some $7000 I can install at my power pole to start and run the entire house. 6Kva usually is adequate for most homes. They are fixed features that look almost like the AC compressor, thus I would not treat them as "personal property".
Yes, add it to the cost approach imho, but unless I can demonstrate that such features add to value explicitly and that addition is sufficiently large to be adequately measurable then I am more likely to "lump sum" with other "site improvements." as a judgment call.
In a poultry farm, lacking such a system is fatal to a contract, therefore would be a "cost to cure" addition to the appraisal. In a home, more nebulous but if the power has been out for over 3 hours in the past 2 years, I bet there is a market reaction to it and it is one hot "selling feature". If you adjuste for "fireplace" then certainly they are going to be as likely to add value as a fireplace in my region.