A BTU, short for British thermal unit, is a basic measure of thermal (heat) energy. One BTU is the amount of energy needed to raise the temperature of 1 pound (0.45 kg) of water by 1° Fahrenheit (0.55° Celsius). In other words, if 16 ounces (0.47 l) of water at 59°F (15°C) were poured into a stovetop pan and the gas burner turned on, it would take 1 BTU to raise the temperature of the water 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 take about 153 BTUs. This non-metric unit of measure is commonly employed only in certain countries — including the US — and is used mainly 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) increase in temperature varies slightly with the starting temperature of the water. There is no universal agreement on what this should be, so the definition varies with place and context. In the US, 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 may be an average over a range of temperatures, or it may 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 in particular, heat, is also sometimes measured in calories, with 1 calorie being 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 is equivalent to 252 calories. In terms of power, 1 British thermal unit is approximately 0.000293 Kilowatt-hours, or a little under one third of a watt-hour.
A single BTU is quite a small unit of energy. Appliances and devices 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 is roughly equivalent to burning one match.
There are other, larger units that are part of the same system. An 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.
Despite its name, the British thermal unit is rarely used in Britain. It is a pre-metric measurement, and as such is employed mainly in countries where that 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 US and Canada, the British thermal unit is the standard measurement of heat output for appliances such as heaters and gas grills, and for fuels such as coal, oil and natural gas. It is also used as a measurement of the cooling effect of refrigerators and air conditioning: these may be given a rating in terms of how many BTUs they can remove from their environment.
The ratings that appear on appliances are actually BTUs per hour, whereas those for fuels are per unit weight, which may be in pounds or tons, or per unit volume, which may be in gallons, cubic feet or barrels. For example, a fan heater may be rated at 34,000 BTU/hr. In the case of fuels, some typical values are 35 million BTUs per ton for coal, 5.6 million per barrel for crude oil, and 1,030 per cubic foot for natural gas.
Calculating Heating and Cooling Requirements
The ability of a source of heat to bring about a given rise 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, a lot 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 given in metric/SI units, such as kilojoules per kilogram, whereas the rating of a heater may be in BTUs per hour. To calculate the energy requirement to heat a room by a certain amount from this information would therefore require finding the specific heat of air, establishing the volume of the room, calculating the weight of the air, and converting the units as required.
To complicate matters still further, the specific heat value for air is normally that for dry air at normal pressure and for a given starting temperature. The actual value will vary with humidity, pressure, and initial temperature. Fortunately, however, there is no need to go to all this trouble: there are tables and simple online calculators that can be used to work out the approximate heating requirements for a room, based on its dimensions, location, the required temperature increase, 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 about 24,000 BTUs per hour.
Calculating Fuel Consumption and Cost
Working out the cost of using a heater is pretty straightforward, if the BTU rating is known. For an electric heater, the rating can simply be converted into kilowatt-hours and the cost per kilowatt-hour obtained from the power company. For an appliance that uses fuel, the rating can be compared with that for the fuel to find out how much it will use over a given period. For example, kerosene has a rating of 135,000 British thermal units per gallon, so a kerosene heater with a rating of 25,000 BTUs will use 1 gallon (3.78 l) of kerosene in 5.4 hours.