Energy Unit Conversions: kWh, Therm, Btu and Gallons

Energy comparisons become confusing when a utility bill uses kilowatt-hours while a fuel supplier bills therms or gallons. The units measure energy, but the amount represented by each differs. This page collects the conversion factors WattWise uses in its calculators and shows how to apply them without implying that fuels provide identical service after equipment losses.

Electricity: watts, kilowatts and kilowatt-hours

A watt is a rate of using energy at a moment; a kilowatt is 1,000 watts. A kilowatt-hour is the energy used by a one-kilowatt load operating for one hour. Convert appliance watts to kilowatts by dividing by 1,000, then multiply by hours. For example, a hypothetical 800-watt device running 3 hours uses 0.8 × 3 = 2.4 kWh. If a hypothetical price is 20 cents per kWh, the energy charge arithmetic is 2.4 × $0.20 = $0.48. Actual bills may include delivery charges, fixed fees, taxes or time-based rates in addition to a simple energy price.

kWh and Btu

EIA lists 1 kWh as 3,412 Btu. To convert kWh into Btu, multiply by 3,412. To convert Btu into kWh, divide by 3,412. One MMBtu means one million Btu, so it is about 293.07 kWh of energy using that conversion. The reverse is useful for heating-cost comparisons: an electric resistance load using 293.07 kWh has roughly one MMBtu of input energy before other system considerations. A heat pump can deliver more heat than the electrical energy it consumes because it moves heat from outdoors; its seasonal COP must be included to estimate delivered heat.

Therms and heating-fuel gallons

One therm of natural gas represents 100,000 Btu. Therefore, 10 therms correspond to one million Btu of fuel input. A gallon of propane is about 91,500 Btu and a gallon of home heating oil is about 138,500 Btu. These are general energy-content conversions. They do not account for combustion efficiency, appliance tuning, distribution losses, or the fact that a gallon delivered to one customer may have slightly different real-world characteristics.

When calculating useful heat from a furnace, multiply fuel input by the equipment's efficiency. A 90% AFUE furnace turns 10 therms (one MMBtu of input) into about 0.9 MMBtu of delivered heat under the rating convention. To supply one MMBtu delivered at that assumed efficiency takes about 1 ÷ 0.90 = 1.111 MMBtu of fuel input. For a heat pump, divide electric input by seasonal COP to express the relationship between electrical energy and delivered heat in simplified form.

Check units before comparing a bill

Read the unit printed beside the measured quantity. Natural gas bills may show therms, cubic feet or another utility-specific measure; do not assume a volume reading is already therms. Oil and propane statements may report gallons, while an electric bill reports kWh. Prices must be on matching units: dollars per gallon cannot be compared directly with cents per kWh. Convert energy first, then include the equipment efficiency and the service being compared.

The WattWise heat-pump calculator uses these factors for natural gas, propane, oil and electricity. Its appliance calculator converts watts to kWh; its EV tool uses kWh per mile. The conversions are not a substitute for a bill's tariff or a heating system's performance data. EIA's conversion calculator is the source of the values shown here. Recheck the source if a future revision changes a factor, and retain enough precision during intermediate steps to avoid rounding errors. Round final displayed costs to cents only after completing the calculation.