Separately, if you have a time-of-use utility plan ... Certain natural gas and propane generators can connect to your home's gas line or propane tank, so there's no need to refill them manually.
Many small gasoline engines can be safely modified to run on natural gas or propane with a kit that ... definitively not the safe way to run your generator on natural gas, but if you ever find ...
“Generators running at full capacity consume significantly more fuel than when operating at partial load. For instance, a generator rated at 20 kW might use about 3.0 gallons of propane per hour ...