Generator Fuel Calculator

A 3,500 to 5,000 W inverter generator running a fridge, lights and a furnace blower (about 200 W on average) burns about 2 gallons of gas a day, and a small 2,500 W inverter about 1. Add a well pump and a window AC and it is closer to 2.5 to 3 gallons. A large open-frame 7,500 to 12,000 W unit burns 7 gallons or more even at light loads, and 12 to 20 at heavy ones. The calculator above gives the number for your exact load and model.

About 17 to 24 hours for a mid-size inverter at a light household load of around 1,000 to 1,500 W, and 6 to 10 hours for a big open-frame unit at a heavy load. Inverter generators slow the engine down at light loads, so they stretch a gallon much further than open-frame models.

Usually 5 to 12 hours. A 20 lb tank holds about 4.6 gallons of propane, and propane carries about 72 percent of gasoline's energy per gallon, so a tank equals roughly 3.3 gallons of gas. At a 1,500 W household load that is about half a day on a mid-size dual fuel inverter.

Gas is usually cheaper per hour. At $3.20 a gallon for gas and $22 to refill a 20 lb tank, propane costs about twice as much for the same energy. Propane wins on storage, since it never goes stale and tanks can sit for years, which is why dual fuel generators are the default recommendation for outages.

Store enough for three days at your load. For a fridge, furnace blower and lights on a mid-size inverter that is about 6 to 10 gallons, and 15 to 20 once a well pump, freezer and window AC join in, kept in 5 gallon cans with fuel stabilizer and rotated every six months. If you want more than three days, add propane tanks or a natural gas hookup rather than more gasoline.

At the same load, a bigger inverter generator often uses less fuel than a smaller one running near its limit, because engines are most efficient in the middle of their range. A big open-frame generator is the exception: it runs at a fixed 3,600 rpm and burns a lot even at light loads. Size for your starting watts, then pick the most efficient model that fits.

Multiply gallons per day by your gas price. At $3.20 a gallon, the Champion 4500 at an 800 W load burns about 3.6 gallons, about $12 a day. The Westinghouse iGen8200TFc burns about 5.1 gallons at 1,500 W, about $16, and about 8.5 gallons at 2,500 W, about $27.

Divide the tank size by the published run time to get gallons per hour at the rated load, then scale by your load divided by that rated load. The iGen5000DFc runs 18 hours on 3.4 gallons at quarter load (975 W), so 0.19 gallons an hour. At 1,500 W that is 0.19 x 1,500 / 975 = about 0.29 gallons an hour, or about 7 gallons a day.

Add each appliance's running watts times the share of time it actually runs. A 150 W fridge cycles about a third of the time, so it averages about 50 W, and a 600 W furnace blower runs about half the time in winter, about 300 W. Starting watts do not matter for fuel, only for sizing the generator.

Yes. An idling engine still burns about half its quarter-load rate, which is the floor this calculator uses. For the iGen5000DFc that is about 0.19 / 2 = 0.09 gallons an hour, roughly 2.3 gallons a day with no load at all. Shut the generator off when you do not need it, for example by running it four hours on and four hours off, which keeps a fridge cold.