I treat them as real property, however I have yet to see a market based measurable value of them. Maybe a slight bump in reconciliation.
Depending on the size and how it's affixed to the property, they can have an impact on value. Did one rural subdivision near a ski slope and just about every house in the development had one. Paired sales back 5 years indicated a 0.3% difference....which I wouldn't bother adjusting for.
Tend to think these are a liability for appraisers, so have a boilerplate comment I add for homes with affixed generators.