How Are Btu Measured?

Typical Measurements

• Energy from food is usually expressed in quantities called calories. Electrical energy is expressed in watts. However, heat energy can be expressed in several ways. Heat energy is expressed as joules in the metric, or International, system and Btu in the English system, but it can also be expressed as calories.

Measuring Btu

• A Btu is equal to 1,055 joules in the metric system. You can also measure Btu as watts. When doing so, one watt-hour equals 3.41 Btu. In addition, you can measure Btu as calories, with one Btu equal to 252 calories. According to the University of Wisconsin, one Btu equals about how much energy is in the tip of just one match stick.

Btu Measurement Defined

• A Btu is officially defined as the amount of heat it takes to raise the temperature of one pound of water by one degree Fahrenheit. However, people often are not dealing with heating water when they need to use Btu to measure energy. So being able to measure Btu in other ways, such as in watts or calories, has practical applications.

Using Btu

• Air-conditioning systems are rated in Btu. If you know how many watts the light bulbs in a building are, you know how much heat they put off and can figure out how many Btu of heat an air conditioning system needs to remove the heat from the lighting in a building. When measuring the amount of energy in natural gas, for instance, therms are often used with one therm equal to 100,000 Btu. If measuring the energy of food, then, for example, 1,000 Btu equals the 252 kilogram calories in an average candy bar, or 80 percent of those in a peanut butter and jelly sandwich, according to the University of Georgia.

Converting Fuel Energy

• In addition, you can convert the average amount of energy in basic fuels to measure that energy content in Btu to make it easier to compare. For example, 1 kilowatt hour of electricity equals 3,413 Btu; 1 therm of natural gas equals 100,000 Btu; 1 lb of coal equals 8,100 to 13,000 Btu and one standard cord of wood equals 18,000,000 to 24,000,000 Btu. Using Btu to compare the energy in fuel is also handy to compare costs. One gallon of gasoline equals 125,000 Btu; 1 gallon of ethanol equals 84,400 Btu; 1 gallon of gasohol (10 percent ethanol, 90 percent gasoline) equals 120,900 Btu and 1 gallon of E-85 (85 percent ethanol, 15 percent gasoline) equals 90,500 Btu.