The calculation

Fuel cost = units per hour × price per unit × hours.

Input What to use Common mistake
Consumption Manufacturer figure at a stated load Using another model’s figure
Price Your fuel supplier’s matching unit price Mixing cubic feet and therms
Hours Planned operating time Assuming every outage lasts the same time
Other charges Separately identified fees Treating fuel-only arithmetic as total ownership cost

A clearly labeled example

Suppose a generator consumes 2 gallons per hour at the selected load and your fuel costs $3 per gallon. Fuel costs $6 per hour, or $144 for 24 operating hours. These are illustrative inputs, not a current fuel-price quote or a claim about any particular generator.

If consumption rises to 3 gallons per hour at a heavier load, the same arithmetic becomes $9 per hour. That is why the load condition belongs beside every estimate.

Match the billing units

For propane, use liquid gallons of usable fuel with liquid-gallon consumption. For natural gas, a specification may state cubic feet per hour while a bill uses a different volume or energy unit. Ask the utility for the applicable conversion and tariff before multiplying. Do not equate a cubic foot with a therm.

The Generac 7327 specifications illustrate how one manufacturer provides different consumption figures at half and full load. Use the sheet for the equipment actually in your proposal.

Fuel cost is only one ownership cost

Keep routine service, repairs, fuel delivery charges and installation costs separate. See maintenance costs and installed-system costs. A comparison is easier to audit when every category and exclusion is visible.

Connect the estimate to runtime

If you also know the usable stored fuel, use the runtime calculator. A fuel-duration estimate does not establish how long the engine may run without inspection or service. Use the equipment manual for that question, and read our extended-outage planning guide.