Propane usage on average is about 130% of gasoline usage of the same generator, but some poor conversion can have the % even higher.
How it can be different - I can compare gasoline models.
The 4500W Generac would use 1 gallon of gas per hr with 1/3 load.
My Honda 3000 will run 6 hr on the same amount of gas, while running AC, or probably couple of days for battery charging and coffee maker.