What this calculator estimates
A generator’s fuel use depends mostly on two things: how big the engine is and how hard you are working it. Even at idle, an engine burns a fixed amount just to keep turning. On top of that, every extra kilowatt you draw costs a roughly fixed amount more fuel. The calculator uses that simple two-part model to estimate an hourly burn rate and how long the fuel you have will last.
I calibrated the model against published run-time figures from real spec sheets, including a small Honda inverter, a mid-size Generac portable, a 22 kW Generac standby on propane and an Onan diesel RV generator. Across those, the estimates land within about 25 percent of the published numbers. Treat the result as a planning figure, not a guarantee.
Tips for a realistic answer
- Use your average load, not your peak. A fridge cycles on and off, lights come and go. For a typical outage, the average is often a third to half of the generator’s rating.
- Propane is sold several ways. A standard 20 lb grill cylinder holds about 4.7 gallons. A 100 lb cylinder holds about 23.6 gallons. Large tanks are usually filled to about 80 percent of their stated size.
- Cold weather matters. Small propane cylinders can struggle to vaporize fast enough in freezing weather, which limits how much the generator can deliver.
- Diesel goes further. Diesel engines are more efficient, which is why RV and commercial units often run on it.
Natural gas
A natural gas generator draws from the utility line, so there is no tank to run dry. Instead the calculator shows the gas flow in cubic feet per hour. Your installer uses that figure to check the meter and the pipe size. Gas piping is licensed work.
Your owner’s manual will have a run-time chart for your model, and that always wins over a general estimate. For more on fuels, see the generator fuel guides. To size the generator first, try the wattage calculator.