Generator Runtime Calculator
Estimate how long a backup generator will run on a given fuel supply, based on fuel consumption rate at the current load level.
Inputs
Consumption varies with load — check the genset's fuel curve at your actual load %, not just full-load spec
Keep a reserve for refueling logistics and to avoid running the tank dry
Usable Runtime (with reserve kept)
18.0hours
Absolute Max Runtime (full tank, no reserve)
20.0hours
Usable Fuel
450.0gal
Reserve Fuel (not counted toward runtime)
50.0gal
Step by step
Values used
Fuel Tank Capacity = 500 gal; Fuel Consumption Rate at Current Load = 25 gal/hr; Fuel Reserve (do not deplete below) = 10
Generator runtime
hours = fuel_capacity_gal × (1 − reserve%) / consumption_gal_per_hr_at_load
Usable Runtime (with reserve kept)
= 18.0 hours
Absolute Max Runtime (full tank, no reserve)
= 20.0 hours
Usable Fuel
= 450.0 gal
Reserve Fuel (not counted toward runtime)
= 50.0 gal
How it works
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.
Formula
Generator runtime
hours = fuel_capacity_gal × (1 − reserve%) / consumption_gal_per_hr_at_load
- F
- Fuel tank capacity in gallons
- r
- Reserve fraction kept unused
- c
- Consumption rate at current load (gal/hr)
Frequently Asked Questions
Why does load level affect fuel consumption rate so much?
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.
Why keep a fuel reserve instead of using the full tank capacity?
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.
How do I plan generator fuel for a multi-day outage?
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.