Generator runtime is tank capacity divided by fuel consumption: runtime (h) = gallons in the tank ÷ gallons per hour at your load. Enter your tank size and the gal/h from the spec sheet at the load you expect. The preset — a 7-gallon tank burning 0.75 gal/h — runs about 9.33 hours, using roughly 18 gallons over a full day.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Runtime on one tank
9.33 h
Continuous use (24 h)
18.0 gal/day
Formula
Runtime in hours = tank capacity (gallons) ÷ fuel consumption (gallons per hour) at the running load. Daily fuel at that load = gal/h × 24. Consumption is not fixed: it rises with load, so a generator at 50% load burns far less than at full output. Use the gal/h figure from the manufacturer's data at the load percentage closest to how you actually run it.
Worked example
A portable generator has a 7-gallon tank and burns 0.75 gallons per hour at about half load. Runtime = 7 ÷ 0.75 = 9.33 hours on one tank. If you ran it continuously at that load, daily use = 0.75 × 24 = 18 gallons. At full load it might burn 1.2 gal/h, cutting runtime to under 6 hours, which is why the load-specific consumption number matters.
Frequently asked questions
How long will a generator run on a tank of gas?
Divide the tank capacity by the fuel-consumption rate at your load. A 7-gallon tank at 0.75 gal/h lasts about 9.3 hours; the same tank at 1.2 gal/h (heavier load) lasts under 6 hours. Inverter generators with an eco/economy mode throttle the engine to the load and stretch runtime considerably. Manufacturers usually quote runtime at 25% or 50% load, so compare quoted figures at the same load percentage.
How much fuel does a generator use per hour?
It depends on size and load. Small inverter units (1–2 kW) burn roughly 0.1–0.3 gal/h, mid-size portables (4–7 kW) about 0.5–1.0 gal/h, and large units more. Consumption climbs steeply near full output. The only reliable number is the manufacturer's gal/h figure at a stated load — enter that here rather than a guess, because fuel use is far from linear with rated wattage.
Does fuel consumption change with load?
Yes, significantly. A generator at 25% load may burn a third of what it uses at full load. Running a large generator to power a small load wastes fuel and can cause wet-stacking in diesel units. For efficiency, size the generator so your typical load sits around 50–75% of its rating, and enter the gal/h that matches that operating point for an accurate runtime estimate.
How do I estimate fuel cost per hour?
Multiply the consumption rate by the local price you pay. If the unit burns 0.75 gal/h, the hourly cost = 0.75 × your current price per gallon. This tool intentionally uses no fixed fuel price — prices vary by region and over time — so you supply the gallons per hour and apply your own current price for the cost. Runtime here is purely the physical tank-divided-by-consumption figure.
How big a fuel supply do I need for 24 hours?
Multiply the gal/h at your load by 24. At 0.75 gal/h that is 18 gallons a day; at 1.0 gal/h it is 24 gallons. Portable tanks rarely hold a full day, so for extended outages plan refueling intervals (let the engine cool first) or connect an extended-runtime tank or a propane/natural-gas line. Always store gasoline safely and never refuel a hot or running generator.
Source: Runtime = tank capacity ÷ consumption; the user enters gal/h at their load (no fuel price hardcoded). · All sources