Generator Fuel Runtime and Cost Calculator

Estimate generator runtime per tank, fuel required, refueling stops, and operating cost for an overnight or multi-day power outage.

Generator and Fuel

Fuel Type

Generator Tank Capacity (gallons)

Fuel in Generator at Start (gallons)

Fuel Consumption (gallons per hour)

Fuel Price Per Gallon ($)

Planning Reserve (%)

Use the fuel-consumption rate listed by the generator manufacturer at the load you expect to run. Fuel use normally increases as generator load increases.

Outage Plan

Generator Hours Per Day

Number of Outage Days

Generator Fuel Estimate

Total Generator Runtime —

Runtime Per Full Tank —

Fuel Required Before Reserve —

Recommended Fuel With Reserve —

Additional Fuel to Obtain —

Tankfuls of Fuel Used —

Estimated Refueling Stops —

Estimated Fuel Cost —

Cost Per Generator Hour —

Fuel consumption varies with generator load, equipment condition, temperature, altitude, fuel quality, and maintenance. Actual runtime may differ.

How Long Will a Generator Run on One Tank?

Divide the generator’s usable fuel-tank capacity by its hourly fuel-consumption rate. A five-gallon generator burning 0.5 gallon per hour has an estimated runtime of 10 hours per full tank.

Runtime per tank = tank capacity ÷ gallons used per hour

How Much Generator Fuel Do I Need?

Multiply the number of generator operating hours by the fuel-consumption rate. If the generator will run for 12 hours per day during a three-day outage, the total planned runtime is 36 hours.

Fuel required = operating hours × gallons used per hour

Why Generator Load Matters

Generator fuel-consumption specifications are commonly listed at a particular load, such as 25%, 50%, or full rated load. A generator powering only a few essential appliances generally burns less fuel than the same generator operating near maximum capacity.

Use the consumption rate that most closely matches your expected load. The Generator Appliance Load Builder can help estimate the wattage of the appliances you plan to run.

Generator Fuel Cost

Fuel cost is calculated by multiplying the planned fuel requirement, including the selected reserve, by the price per gallon. The cost of fuel already on hand is included because that fuel is still consumed during operation.

Gasoline, Diesel, and Propane Runtime

Different generator models and fuel types have different consumption rates. Dual-fuel and tri-fuel generators may also produce different amounts of power depending on the fuel being used. Always use the specifications for the generator model and selected fuel.

Propane-container capacity may be advertised in pounds rather than gallons. Confirm the usable gallons of liquid propane and the manufacturer’s expected runtime before entering the values.

Why Include a Fuel Reserve?

A planning reserve provides extra fuel for longer outages, heavier loads, refueling delays, and consumption that exceeds the manufacturer’s estimate. The reserve is added to the calculated fuel requirement.

Generator and Fuel Safety

Never operate a fuel-powered generator indoors, inside a garage, in a basement, or near doors, windows, or vents. Generator exhaust contains carbon monoxide and can be deadly.

Store gasoline, diesel, and propane only in approved containers and locations. Follow local fire codes, manufacturer instructions, fuel-storage limits, and safe refueling procedures. Allow a hot generator to cool before adding liquid fuel.

Planning Estimate Only

This calculator provides a planning estimate and does not replace generator specifications, fuel-container ratings, safety instructions, or professional guidance.