A BTU is a measure of thermal energy used to rate the heat output of fuels and appliances. It is the amount of energy required to raise the temperature of 1 pound of water by 1°F. One BTU is equal to between 1,054.35 and 1,059.67 joules. The British thermal unit is mainly used in countries where the metric system has not been fully adopted, and is the standard measure of heat output for appliances and fuels in the US and Canada. It is also used to calculate heating and cooling requirements and fuel consumption costs.
A BTU, short for British thermal unit, is a basic measure of thermal energy (heat). One BTU is the amount of energy required to raise the temperature of 1 pound (0.45 kg) of water by 1° Fahrenheit (0.55° Celsius). In other words, if you pour 16 ounces (0.47 L) of 15°F (59°C) water into a pan and turn on the gas stove, it would take 1 BTU to raise the water temperature to 60°F (15.6°C). If the pan was left on the gas flame, the water would eventually reach the boiling point of 212°F (100°C), which would require about 153 BTUs. This non-metric unit of measurement is commonly used in only a few countries, including the United States, and is primarily used to rate the heat output of fuels and appliances.
Definition and conversions
The precise amount of heat required to achieve a 1°F (0.55°C) temperature rise varies slightly with the initial temperature of the water. There is no universal agreement on what it should be, so the definition varies by location and context. In the United States, a starting temperature of 59°F (15°C) is generally used, but in Canada it is 60°F (15.6°C). In other cases, it can be an average over a range of temperatures or it can be equated to a specific value in joules, which are the SI units for energy.
Depending on the definition used, 1 BTU is equal to between 1,054.35 and 1,059.67 joules. Energy, and especially heat, is also sometimes measured in calories, with 1 calorie representing the amount of heat required to raise the temperature of 0.035 ounces (1 gram) of water by 1.8°F (1°C). One BTU equals 252 calories. In terms of power, 1 British thermal unit equals approximately 0.000293 kilowatt-hours, or just under a third of a watt-hour.
A single BTU is a rather small unit of energy. Appliances and appliances that have British thermal unit ratings often show values that are in the thousands or tens of thousands. To get an idea of how much energy a single unit represents, it’s roughly equivalent to burning a match.
There are other larger units that are part of the same system. One MBTU is 1,000 British thermal units, but is not often used because the prefix ‘M’, in SI units, normally represents one million, which causes confusion. An MMBTU is sometimes used to represent one million of these units. A therm is 100,000 BTUs.
Use
Despite its name, the British thermal unit is rarely used in Great Britain. It is a premetric measure, and as such it is mainly used in countries where this system has not been fully adopted. In the world of science, the joule is the unit normally used to represent energy, but in some countries, notably the United States and Canada, the British thermal unit is the standard measure of heat output for appliances such as stoves and gas grills. gas, and for fuels such as coal, oil and natural gas. It is also used as a measure of the cooling effect of refrigerators and air conditioning: these can be scored in terms of how many BTUs they can remove from their environment.
The ratings that appear on appliances are actually BTUs per hour, while those for fuels are per unit of weight, which can be in pounds or tons, or per unit of volume, which can be in gallons, cubic feet, or barrels. For example, a fan heater may be rated at 34,000 BTU/hour. In the case of fuels, some typical values are 35 million BTU per ton for coal, 5.6 million per barrel for crude oil, and 1,030 per cubic foot for natural gas.
Calculate heating and cooling requirements
The ability of a heat source to cause a given increase in the temperature of a substance depends not only on the energy of the heat source, but also on the specific heat of the substance. Different materials can have very different specific heats; for example, much more energy is required to heat water than to heat metals. For a heater, the substance in question is normally air. Specific heats are usually expressed in metric/SI units, such as kilojoules per kilogram, while a heater’s rating can be in BTUs per hour. To calculate the energy requirement to heat a room by a certain amount from this information it would then be necessary to find the specific heat of the air, establish the volume of the room, calculate the weight of the air and convert the units as needed.
To further complicate matters, the specific calorific value of air is normally that of dry air at normal pressure and for a given starting temperature. Actual value will vary with humidity, pressure and initial temperature. Luckily, though, there’s no need to go to all that trouble: there are tables and simple online calculators that can be used to calculate the approximate heating needs of a room, based on its size, location, required temperature rise and how well insulated it is. For example, heating a typical 1,000 square foot (92.9 square meter) well-insulated home in Boston during the winter might require approximately 24,000 BTUs per hour.
Calculation of fuel consumption and costs
Calculating the cost of using a heater is pretty easy, if you know the BTU rating. For an electric heater, the rating can simply be converted to kilowatt hours and the cost per kilowatt hour obtained from the power company. For an appliance that uses fuel, the rating can be compared to that of the fuel to find out how much it will use in a given period. For example, kerosene has a rating of 135,000 British thermal units per gallon, so a kerosene heater with a 25,000 BTU rating will use 1 gallon (3.78 l) of kerosene in 5.4 hours.
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