Realistically, having a generator may increase a properties appeal to the buyer but as far a increasing the value, it would be minimal. Similar to a golf cart.....how much would one of these increase a home's value?
I think that having a generator increases the value of a home IF you live in an area where there are frequent storms and power outages. I don't think it would be a very significant value though. I live and work as a Realtor on Cape Cod and I don't think that it would add much if any value here on the Cape.
Also, it depends very much on the buyer as some buyers may require or prefer a home with a generator. Hope that helped and good luck to you!
-Sunny Fellman, Realtor