Generator Fuel Calculator

It depends on the size of the unit and how hard it is working. A typical 22 kW air-cooled home standby generator burns somewhere around 2.3 gallons an hour at half load and about 3.7 gallons an hour at full load. Smaller units use proportionally less — a 10 kW might sit near a gallon an hour at half load — and large liquid-cooled units use considerably more. The calculator works this out from your rated size, engine type and load level rather than quoting a single average, because the load is what moves the number most.

No, and this is the most common surprise on a first fuel bill. The engine has to spin at a fixed speed to hold 60 Hz output whether the house is drawing a lot or a little, so a base amount of fuel is consumed before any of it turns into useful power. Cutting the load in half typically cuts fuel by only around a third — the unit still burns roughly 62% of its full-load rate. The practical consequence is that cost per kilowatt-hour is worst when the generator is barely loaded, which is why running a small, lightly loaded unit can feel disproportionately expensive.

A 500-gallon tank is filled to about 80% for safety, so you have roughly 400 usable gallons rather than 500. Running a 22 kW unit at half load, that works out to about 170 hours — a bit over seven days of continuous running — dropping to around 108 hours if the generator is carrying full load the whole time. Two things stretch it: shedding large electric loads, and letting the unit cycle off overnight if your transfer setup allows it. The calculator applies the 80% fill automatically and shows runtime at every load level so you can see both ends of the range.

Each wins on something different. Natural gas is usually cheaper per hour to run and never runs out, since it comes through the utility line — but the same engine produces roughly 10% less power on gas than on propane, and a gas line can be interrupted by the same event that took out the power, particularly in an earthquake. Propane stores on your property, holds more energy per unit, and lets the unit make its full rated output, but you are limited by tank size and by how quickly a supplier can reach you after a regional storm. If you already have a gas line, that is usually the simpler choice; if you are rural or want independence from another utility, propane is worth the tank.

For a 22 kW unit at half load on propane at around $2.90 a gallon, expect roughly $6 to $7 an hour, which lands near $160 for a full 24 hours of continuous running. On natural gas the same day usually costs less. Multi-day outages add up quickly, and that is worth knowing before an ice storm rather than after — a week at that rate approaches the cost of a small appliance. Enter your own fuel price and outage length in the calculator, since propane prices swing seasonally and vary a lot by region and delivery volume.

Standby generators run a weekly self-test — usually 5 to 20 minutes — to circulate oil, charge the battery and confirm the unit will actually start when it is needed. It runs unloaded, so it burns at the engine’s base rate, but over 52 weeks it still adds up to a real if modest line on your fuel bill, which the calculator totals for you. Do not disable it. A generator that has not run in months is the one that fails on the night of the storm, and most manufacturers require the exercise cycle to keep the warranty intact. If the timing bothers you, most controllers let you move it to a weekday morning.