Calculate how long a backup generator will run on available fuel at a given load.
Generator runtime is fuel tank capacity divided by the consumption rate at the actual operating load — not the generator's full-load or no-load spec, since fuel burn scales with electrical load and using the wrong point on the fuel curve significantly skews the estimate. Subtracting a reserve fraction before dividing accounts for refueling lead time during an extended outage and avoids running the tank fully dry, which risks drawing sediment into the fuel system.
Generator runtime
hours = fuel_capacity_gal × (1 − reserve%) / consumption_gal_per_hr_at_load
Diesel and gas generators consume fuel roughly proportional to electrical output, but not linearly — at very light loads efficiency drops (a genset run at 20% load can burn disproportionately more fuel per kWh delivered than one at 75-80% load), so always use the manufacturer's fuel consumption curve at your actual expected load, not the full-load or no-load figures.
During an extended outage, refueling trucks may be delayed by the same event causing the outage (regional power failures often mean regional fuel delivery delays too) — keeping a 10-20% reserve buys extra runtime margin, and never running the tank fully dry avoids drawing accumulated sediment into the fuel filters and injectors.
Calculate usable runtime per full tank, then plan a refueling schedule so the tank is topped off before it drops below your reserve threshold — for critical facilities, this typically means a service contract with guaranteed refueling response times built into the outage response plan, not just a single large tank.