Generator Fuel Runtime and Backup Duration
How long a standby generator runs on its fuel, the fuel-supply companion to generator-sizing and battery-runtime: runtime = usable fuel / consumption, where usable = tank x usable% (default 90%). A 100 gal tank at 3.0 gph runs 30 hr (1.25 days); for a 72-hour outage design basis it needs 216 gal of fuel and a 240 gal tank, so the 100 gal tank does not meet it. Enter the consumption at load from the genset data plate (a diesel burns ~0.05-0.08 gal/hr per kW at load). Liquid-fuel-tank case; a utility gas feed has no tank limit. A planning aid; the published fuel curve and the AHJ's fuel-storage rules govern.
Formula and source
usable = tank x usable%/100; runtime_hr = usable / consumption_gph; runtime_days = runtime_hr/24; for a target: fuel = consumption_gph x target_hr, tank_needed = fuel / (usable%/100).
Generator fuel runtime from the fuel supply and consumption, first-principles; the genset manufacturer's published fuel-consumption curve governs the rate.
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