What is watt hour?

A watt hour is a unit of energy equal to the power of one watt sustained for one hour. It is used to measure small amounts of electrical energy consumption, particularly in battery capacities and portable electronics.

Real-world uses

Watt-hours measure the energy stored in or consumed from batteries, small electronics, and portable power stations. A smartphone battery holds roughly 10–15 Wh; a laptop battery 40–100 Wh. Solar panel output for small installations is often quoted in Wh per day.

History

The watt-hour emerged as a practical energy unit in the late 19th century with the commercialisation of electric power distribution. It offered a convenient scale between the joule (too small) and kilowatt-hour (too large) for everyday portable energy applications.

Common mistakes

Confusing watt-hours (energy) with watts (power). A device rated at 5 W running for 3 hours consumes 15 Wh. Also, treating Wh and mAh as interchangeable without knowing voltage: Wh = mAh × V ÷ 1000.

What is therm (US)?

A US therm is a unit of energy equal to 100,000 BTU. It is used by gas utility companies in the United States as the standard billing unit for natural gas consumption.

Real-world uses

The therm is used in the United States and United Kingdom for natural gas billing. One therm equals 100,000 BTU (approximately 29.3 kWh). Residential gas bills in the US often list consumption in therms. It is also used in industrial gas metering and procurement contracts.

History

The therm was introduced as a practical unit for natural gas billing in the early 20th century, when gas supply companies needed a convenient large-scale energy unit for residential and commercial customers. It consolidated prior inconsistent measurement practices in the gas industry.

Common mistakes

Confusing the therm with the BTU—1 therm = 100,000 BTU. Also, the US therm and the UK therm differ slightly (the UK therm is based on 105.5 MJ, while the US therm is exactly 100,000 BTU ≈ 105.48 MJ). Not to be confused with the thermie (a metric unit of heat).

When is this conversion used?

Converting between watt hour and therm (US) is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region.

Worked examples

1 watt hour = 0.00003412 therm (US)

1 therm (US) = 105,506,000 joule

How to convert watt hour to therm (US)

To convert watt hour to therm (US), multiply the value by 0.00003412.

To convert therm (US) back to watt hour, multiply by 29,307.22222222.

Measurement standards

The joule is the SI derived unit of energy, defined as one kilogram-metre-squared per second-squared (kg·m²/s²). The kilowatt-hour, equal to exactly 3.6 megajoules, is accepted for use with the SI by the BIPM for practical electricity metering.

Did you know?

A single bolt of lightning releases roughly 1 to 5 gigajoules of energy, yet only a small fraction reaches the ground as electrical current. Most of the energy dissipates as light, heat, and the thunder shockwave.

Quick reference: watt hour to therm (US)

watt hour	therm (US)
0.1	0.00000341
0.5	0.00001706
1	0.00003412
2	0.00006824
5	0.00017061
10	0.00034121
25	0.00085303
50	0.00170606
100	0.00341213
250	0.00853032
500	0.01706064
1,000	0.03412128

Common values

	watt hour	therm (US)
A food calorie (1 kcal)	1.16222222 watt hour	0.00003966 therm (US)
A AA battery	2.6 watt hour	0.00008872 therm (US)
Boiling a kettle	116.66666667 watt hour	0.00398082 therm (US)
Daily human diet (2000 kcal)	2,324.44444444 watt hour	0.07931302 therm (US)
1 gallon of gasoline	36,600 watt hour	1.24883893 therm (US)

Available Energy units

More watt hour conversions

Assumption: calorie means thermochemical calorie, and therm means US therm.