I have a Generac 6000 watt something in the shed, it probably only has 5 hrs on it....fuel is drained and will stay that way, it is not to be used except emergencies, I can plug it straight into a box to power the house, but main reason is in case of a forest fire takes out the powerlines I can still run my pump for fire protection...

 

I keep my 15K unit filled (15 gal), but I added Sta-Bil to the fuel, shook it up to keep the fuel from degrading, then I fire it up for 1/2 hour once every two months.
Then I turn the fuel petcock off and run it until it conks out.
That way I don't have to find enough gas to use it when the doo-doo hits the fan and I NEED the generator.
Lord knows, if I really NEEDed the generator, it'd be the middle of the night, and the can would be empty.

That procedure has been working fine for me. YMMMV

The 16k unit is fueled via a 500 gal propane tank buried in the backyard.
That unit has an "excercise" setting that fires it up automatically once a week and runs it for 20 minutes to keep the battery charged.