Generator Runtime
Calculate how long a backup generator will run on available fuel at a given load.
About Generator Runtime
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)
Worked example
With the default values below, Generator Runtime returns Usable Runtime (with reserve kept) of 18.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Fuel Tank Capacityminimum 1 gal
- 500 gal
- Fuel Consumption Rate at Current Loadminimum 0.1 gal/hr
- 25 gal/hr
- Fuel Reserve (do not deplete below)accepts 0–50
- 10
Results
- 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
Step by step
- 1Values used
- 2Generator runtime
- 3Usable Runtime (with reserve kept)= 18.0 hours
- 4Absolute Max Runtime (full tank, no reserve)= 20.0 hours
- 5Usable Fuel= 450.0 gal
- 6Reserve Fuel (not counted toward runtime)= 50.0 gal
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.