Generator Runtime Calculator
How long a generator runs on the fuel you have — from its fuel use, its spec-sheet runtime or its size and your load — in hours and days, with runtime at 25–100% load, fuel to store for an outage, containers needed and fuel cost per hour, day and kWh.
How it works
Runtime = fuel available ÷ fuel use per hour · From a spec sheet: use = tank ÷ rated runtime, scaled by (c₀ + (1 − c₀) × your load) ÷ (c₀ + (1 − c₀) × rated load) · From size: full-load use ≈ kW × 0.17 gal/kWh (gasoline, conventional), 0.15 (inverter), × 1.31 for propane, 0.075 for diesel · Days = runtime ÷ hours a day
Generator runtime is how much fuel you have divided by how fast the engine burns it at your load. If you know the fuel use per hour, that is all you need. Most spec sheets instead give a runtime on a full tank at 25% or 50% load; dividing the tank by that runtime gives the fuel use at that load. Fuel use does not fall in proportion to load: an engine burns a fixed share even with nothing plugged in (more for open-frame generators that run at constant speed, less for inverters that throttle down), so the calculator scales the rate along a straight-line part-load curve. Without a spec sheet it estimates full-load fuel use from the generator’s size and fuel type. It also plans the fuel to store for an outage, the containers that takes and the running cost.
Worked example
20 L at 1.5 L/h runs about 13 hours 20 minutes; at 8 hours a day that is 12 L a day and 1.7 days. From a spec sheet instead: a 3,500 W conventional generator with a 15 L tank rated for 10 h at 50% load uses 1.5 L/h at 1,750 W. At a 1,500 W load (43%) it uses 1.5 × (0.35 + 0.65 × 0.429) ÷ (0.35 + 0.65 × 0.5) = 1.40 L/h, so 20 L lasts about 14 hours 19 minutes.
Sources: Generator Source — diesel and natural gas fuel consumption by load · U.S. EIA — heat content of fuels (BTU per gallon) · U.S. CPSC — What to know about generators and carbon monoxide
Frequently asked questions
Does load change fuel use?
Yes, but not in proportion. A generator at half load uses roughly 60–70% of its full-load fuel, and even at no load it burns a fair amount, especially an open-frame generator that runs at constant speed. Inverter generators slow the engine down at light loads, so they save the most when you run only a few appliances.
How do I find my generator’s fuel use?
Look for "run time" on the spec sheet, e.g. "10 hours at 50% load" with a 4-gallon tank. Divide the tank by the hours for the fuel use at that load. Choose the spec-sheet method here and enter both numbers; the calculator scales it to your actual load.
How much fuel should I store for an outage?
Fuel per day times the days you want to cover, plus a margin of 10–20%. Store gasoline in approved cans, away from the house, with a stabilizer, and use it within a few months. Propane keeps indefinitely.
Does propane give less runtime than gasoline?
Yes. A gallon of propane holds about 91,500 BTU against about 120,000 for gasoline, so a generator burns roughly 30% more gallons of propane for the same output. Propane also derates the generator by about 5–10%.
Can I run a generator overnight?
Yes, if it is outdoors, away from windows and doors, and you have working CO alarms inside. Many portable tanks will not last a whole night at moderate load, so check the runtime here and refuel only after the engine has cooled.
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This calculator is for educational purposes. Double-check important results before acting on them.