Generator Sizing & Fuel Storage Calculator

To size a backup generator, sum continuous running Watts and add a 25% safety headroom to prevent harmonic distortion (THD) and thermal overload. An inverter generator running at 50% load burns approximately 0.25 to 0.35 gallons of gasoline per hour (or ~1.35× that volume in liquid propane). Crucially, small air-cooled generator engines require an oil change every 50 to 100 operating hours; running a generator continuously for 14 days without oil changes causes permanent internal engine seizure.