GeneratorSizer

kW to kWh calculator

Published October 3, 2026.

Built by Jeremy Panasuk.

A kilowatt is a rate. A kilowatt-hour is that rate held for time. The generator sizer outputs the rate. This box only multiplies.

Quick answer: EIA defines a kilowatthour as 1 kilowatt (1,000 watts) of power expended for 1 hour. One kWh equals 3,412 Btu. A 3,500 W recommendation held for 8 hours is 3.5 kW × 8 h = 28 kWh, which is 28 × 3,412 = 95,536 Btu.

Where the 3,500 W comes from

The 3,500 W is not a new measurement. It is the recommendation on best generator for a food truck: Honda's Energy Star fridge at 192 W running / 1,200 W start, a 4-cup coffee maker at 600 / 600, and a 650 W microwave at 1,000 / 1,000. Running total 1,792 W, extra surge 1,008 W, peak 2,800 W, 20% headroom, rounded up to the next 500 W. The generator size calculator stops at watts, gallons per hour, and hours on a tank. It does not print kilowatt-hours. EIA's definition is the step that turns those watts into energy.

Eight hours is the calculator's default "hours per day" field, used here only as the time in EIA's definition. It is not a claim that the food truck draws 3,500 W for eight continuous hours. The recommendation is a size, not a measured load.

Convert watts and hours

The factor 3,412 Btu per kWh is EIA's. The arithmetic is watts ÷ 1,000 × hours, then × 3,412.

A common mistake

Calling 28 kWh a 28 kW generator, or skipping the sizing steps and multiplying a single running watt by hours as if that product were the machine to buy. EIA's conversion does not add surge, and it does not apply the 20% headroom. Those happen in sizeGenerator() before you have a watt number worth converting. Dollar comparisons of fuel versus a standby install stay on five-year cost. That page prices outage hours. This page does not.

Limits, and when to call a pro

This conversion does not size a service, a transfer switch, or a generator. Permanent installation takes a licensed electrician and a permit where local code requires one. If the watt input is a nameplate you have not run through the sizer, the kilowatt-hours inherit that gap.

Compiled with AI assistance from the sources below. Spot an error? Email [email protected].

Sources