Generator Fuel Use and Runtime Estimator

Manufacturers publish a generator's fuel use at two points, half load and full load. The estimate draws a straight line between them for the load you enter. Below half load it extends the same line down to 25 percent, and never goes below half of the 50 percent figure, because a running engine still burns fuel at a light load.

Natural gas is shown in cubic metres and propane in litres, with the spec sheet's unit beside it. One cubic foot is 0.028317 cubic metres and one US gallon is 3.78541 litres. Propane vapour figures are turned into liquid on the standard values: one US gallon of liquid propane gives about 36.4 cubic feet of vapour at 60 degrees Fahrenheit, and one pound of propane is about 0.236 US gallons.

A propane tank lasts its usable volume divided by the use per hour. The usable volume is the tank size times the fill level; 80 percent is the usual fill. On natural gas there is no tank: the runtime is set by the gas supply.

A generator's natural gas rating is below its propane rating on every brand we checked: size on the fuel you will use. The estimate shows fuel, not what it costs. A licensed electrician's load calculation, and a licensed gas fitter for the gas connection, decide the real figures on site.