Therms to Watt-hours (thm to Wh)

1 therm equals 29,300.11111 watt-hours (rounded).

1 therm = 29,300.11111 watt-hours

How to convert therms to watt-hours

To convert therms to watt-hours, multiply the number of therms by 29,300.11111. The factor is rounded to 10 significant digits. To go the other way, multiply watt-hours by 0.00003412956341 (rounded).

watt-hours = therms × 29,300.11111

Example: 10 therms = 293,001.1111 watt-hours.

The 'calorie' on food labels is really a kilocalorie. The thermochemical calorie (4.184 J) is used here unless marked IT.

About these units

Therm (thm)

US customaryTraditional unit (still used in some regions)

100,000 BTU (about 105.48 MJ). US natural gas bills are measured in therms.

Gas bills gave the therm its place in daily life. It is defined as 100,000 British thermal units, so its size depends on which BTU is meant. The figure used here, 105,480,400 joules or about 29.30 kWh, is the US therm, built on a BTU defined at 59 degrees Fahrenheit. The European Community version, based on the ISO BTU, is 105,506,000 joules, and the UK value is about 105,505,585 joules.

In the United States, natural gas is billed in therms or in CCF, hundreds of cubic feet. Pipeline companies adopted the decatherm, 10 therms or a million BTU, around 1972 to simplify invoicing, and roughly 1,000 cubic feet of gas holds about one decatherm.

Britain moved the other way. Regulations replaced therms with joules from 1999, retail customers now see kilowatt-hours, and only the wholesale market still quotes pence per therm. The EIA lists one therm as 100,000 Btu, and the Btu itself is the heat needed to warm a pound of water by one degree Fahrenheit.

1 therm = 105,480,400 joules (exactly).

Sources: Wikipedia: Therm; EIA: British thermal units (Btu); Wikipedia: British thermal unit

Watt-hour (Wh)

Metric (SI)Current unit

Exactly 3,600 joules, the energy used by a one-watt device in an hour. Battery capacities are quoted in watt-hours.

Pair a power unit with a unit of time and you get the watt-hour: one watt sustained for one hour, which works out to 3,600 joules. The watt itself was adopted at the second International Electrical Congress in 1889, in the same session that adopted the joule, so the two units share a birthday even though the watt-hour is the one people read off batteries.

Battery capacity is where it earns its keep. Multiplying amp-hours by the battery voltage gives an estimate of watt-hours, so a 2,500 mAh lithium-ion cell at 3.7 V holds roughly 9.25 Wh. That is why a 500 mA USB device runs about 3.7 hours on it rather than five: the cell voltage, not the 5 V from the port, sets the energy.

For scale, an alkaline AA battery holds about 9 kJ, which is 2.5 Wh. A 12 W LED lamp left on for an hour uses 12 Wh, and a 40 W appliance running for 25 hours uses 1,000 Wh.

1 watt-hour = 3,600 joules (exactly).

Sources: Wikipedia: Watt-hour; Wikipedia: Joule; Wikipedia: Orders of magnitude (energy)

Related conversions

See all energy units.