Generator Runtime Calculator
Runtime is tank capacity divided by consumption rate, but consumption is not a single number — it varies strongly with load. A set at 25% load burns far less per hour than at 75%, and using the full-load figure for a lightly loaded standby set will badly understate how long it runs. The number that matters for a standby installation is usually how long it lasts at the actual connected load, not at nameplate.
Run the numbers
The working calculator is free and needs no signup.
The formula
| Tank capacity | Usable fuel volume, which is less than nominal capacity once the unusable heel is excluded |
| Consumption rate | Gallons per hour at the expected load, from the genset spec sheet |
Worked example
A 500 kW diesel set with a 1,000 gallon tank, consuming 26 gal/hr at 75% load:
Runtime = 1000 / 26 = 38.5 hours
At 25% load the same set might burn 11 gal/hr, giving about 90 hours — more than twice as long from the same tank.
Which standard governs this
Emergency and standby system requirements come from NFPA 110, which classifies systems by how long they must run without refuelling. On-site fuel storage is governed by NFPA 30 and local fire code.
What this calculation does not account for
Depends entirely on the consumption figure used, which should come from the manufacturer's data at the actual load, not a rule of thumb. Ignores the unusable heel at the bottom of the tank, ambient temperature effects, and fuel-quality degradation in long-idle standby installations — which is a more common cause of failure than running out.
Common mistakes
Using nameplate consumption for a lightly loaded set. Treating nominal tank capacity as usable. Sizing storage for runtime without checking what the local fire code allows to be stored on site. Ignoring fuel polishing on a tank that sits full for years.