| The British thermal unit (BTU) is a non-metric unit of energy, used in the
United States and, to a lesser extent, the UK (although the BTU is extremly rare in the UK now). The SI unit is
the joule (J), which is used by most other countries. A BTU is defined as the amount of
heat required to raise the temperature of one pound avoirdupois of water by one degree Fahrenheit; specifying the temperature
range over which this happens leads to a number of slightly different BTU values, varying over a range of about 0.5%:
| Name |
Value (J) |
Notes |
| 39°F |
≈ 1059.67 |
Uses the calorie value of water at its maximum density (39.1–39.2°F) |
| Mean |
≈ 1055.87 |
Uses a calorie averaged over the 32°F to 212°F range |
| IT |
≡ 1055.05585262 |
The most widespread BTU, uses the International [Steam] Table (IT) calorie, itself defined for water at 14.5°C (58.1°F) by
the Fifth International Conference on the Properties of Steam, held in London in
July 1956. |
| 59°F |
≡ 1054.804 |
Chiefly American. Uses the 15°C calorie, itself defined as exactly
4185.5 J (Comité international 1950; PV, 1950, 22, 79-80) |
| 60°F |
≈ 1054.68 |
Chiefly Canadian |
| 63°F |
≈ 1054.6 |
Possibly apocryphal |
| ISO |
≡ 1054.5 |
ISO 31 Quantities and units (?) |
| Thermochemical |
≡ 1054.35026444 |
(Calculated from 9489.152 380 4 ÷ 9) Uses the thermochemical calorie of exactly 4184 J |
The BTU is often used to describe the heat value of fuels, or the heating and cooling capacity of a system (such as a barbecue
grill).
One BTU is approximately:
The BTU/hour (BTU per hour) is the unit of power most commonly associated with the BTU.
- 1 horsepower is approximately 2500 BTU/hour
- 1 watt is approximately 3.4 BTU/hour
- 1000 BTU/hour is approximately 290 W
A unit called the quad (short for quadrillion) is defined as 1015 BTUs, which is about 1.055×1018 joules, and the
therm is defined in the United States and European Union as 100,000 BTU –
but the U.S. uses the BTU59°F whilst the E.C. uses the BTUIT.
The BTU should not be confused with the Board of Trade
Unit (B.O.T.U.), which is a much larger quantity of energy.
|
