Home Generator Fuel Cost Calculator

A generator fuel bill depends on five facts, and this calculator asks for each one in order.

1. Enter the generator size in watts. Use the running watts on the nameplate, not the starting watts. A common home portable is 7,500 W.
2. Enter the load as a percent of that size. Most homes run at 30 to 60 percent during an outage.
3. Choose the fuel type, gasoline, diesel or propane. Each fuel burns at a different rate per kilowatt-hour.
4. Enter the fuel price per gallon and the hours you run the generator each day.
5. Press Calculate, or press Enter. The fuel cost per day appears first and large, with the power used, the fuel used per day, the cost per hour and the cost per week below.

The calculator turns load into power, power into energy, energy into fuel and fuel into money. The consumption rates are planning rates from published generator fuel charts. Gasoline burns about 0.10 gallons per kilowatt-hour, diesel about 0.07 and propane about 0.19. The US Energy Information Administration lists the heat content behind those rates, with gasoline at 120,214 BTU per gallon, diesel at 137,381 and propane at 91,452.

Load watts = generator size in watts x load percent / 100. Power used = load watts / 1,000, in kilowatts. Energy per day = power used x hours run per day, in kilowatt-hours. Fuel used per day = energy per day x gallons per kilowatt-hour for your fuel. Fuel cost per day = fuel used per day x fuel price per gallon. Cost per hour = fuel cost per day / hours run per day. Cost per week = fuel cost per day x 7. Our fuel consumption converter can change gallons per hour into other units if your manual quotes the burn rate differently.

A homeowner runs a 7,500 W portable generator at 50 percent load on gasoline at $3.50 per gallon for 8 hours a day.

1. Load watts: 7,500 x 50% = 3,750 W.
2. Power used: 3,750 / 1,000 = 3.75 kW.
3. Energy per day: 3.75 kW x 8 hours = 30 kWh.
4. Fuel used per day: 30 kWh x 0.10 gallons per kWh = 3.0 gallons.
5. Fuel cost per day: 3.0 gallons x $3.50 = $10.50.
6. Fuel cost per hour: $10.50 / 8 hours = $1.31.
7. Fuel cost per week: $10.50 x 7 = $73.50.

As a second check, a 10,000 W diesel generator at 75 percent load, $4.00 per gallon, run 12 hours a day, uses 6.3 gallons and costs $25.20 a day, which is $2.10 an hour and $176.40 a week.

The same generator, a 7,500 W portable at 50 percent load for 8 hours a day, costs a different amount on each fuel. Gasoline at $3.50 a gallon uses 3.0 gallons and costs $10.50 a day. Diesel at $4.00 a gallon uses 2.1 gallons and costs $8.40 a day, because diesel carries more energy per gallon. Propane at $2.50 a gallon uses 5.7 gallons and costs $14.25 a day, because propane carries about a quarter less energy per gallon than gasoline.

Compare fuels by cost per kilowatt-hour, which is the fuel price times the gallons per kilowatt-hour. At these prices gasoline costs $0.35 per kilowatt-hour, diesel $0.28 and propane $0.48. Diesel wins here, but the ranking follows your local prices. Propane stores for years without going stale, while gasoline goes stale in three to six months. Our fuel economy calculator can help you compare fuel costs in a different setting.

Load moves the bill more than anything else. On gasoline at $3.50 a gallon, the same 7,500 W generator run 8 hours a day costs $5.25 at 25 percent load, $10.50 at 50 percent, $15.75 at 75 percent and $21.00 at full load. Shedding a big load such as an electric water heater or a dryer is the fastest way to cut the bill.

Hours matter just as much. At 50 percent load the worked example costs $10.50 for 8 hours, $15.75 for 12 hours and $31.50 for a full 24 hours. A 12,000 W unit at full load on gasoline uses 28.8 gallons a day and costs $109.44 at $3.80 a gallon. One limit of this calculator is that real engines burn extra fuel just to keep turning, so at light loads the true burn runs higher than this model shows. A 7,500 W gasoline portable at half load burns about 0.67 gallons an hour in manufacturer fitted data, against 0.375 gallons an hour here, so check your spec sheet before you buy fuel for a long outage.

Run only the loads you need. Fridges, lights, internet and phone chargers keep a home livable near 30 percent load, and each large appliance you switch off drops the load percent and the fuel bill with it. An inverter generator also helps at light loads, because it slows the engine when demand drops, while a fixed speed generator keeps turning at 3,600 rpm whatever the load.

Buy fuel before the storm, not during it, and store gasoline with stabilizer and rotate it through your car twice a year. A 20 pound propane cylinder holds about 4.7 gallons, and at the propane rate above it lasts under a day on a 7,500 W generator at half load, so count cylinders before an outage. If you are still choosing a unit, our home generator installation cost calculator prices the standby unit itself, from size and fuel type to labor.

Never run a generator indoors, in a garage or within 20 feet of a window or door. Carbon monoxide has no smell and kills in minutes, so place the unit outdoors and keep working CO alarms inside the home.