Energy Terms and Energy Unit Conversions

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Definitions:

• British Thermal Unit (BTU) - The quantity of heat energy required to raise the temperature of 1 pound of liquid water by 1 degree Fahrenheit at the temperature at which water has its greatest density (approximately 39 degrees Fahrenheit). As a point of reference, one gallon of gasoline contains about 124,000 BTUs. In metric units of heat energy, one BTU is approximately equal to 250 calories.
• Energy - The capacity to perform work. This can be defined either as the potential amount of work that can be performed (potential energy) or as the amount of work performed through physical motion (kinetic energy). The standard scientific units of measure for energy are joules, although the typical units used in the electrical power industry are BTUs for thermal energy and kilowatt-hours for electical energy.
• Energy Efficiency - The ratio of work performed by a system to the amount of energy input. In the case of electrical power generation sources, efficiency can be measured as a ratio of the electrical energy produced by the source to the energy content of its fuel. For example, if a diesel generator burns one gallon of diesel fuel to produce 10 kWh of electrical energy and one gallon of diesel fuel countains 40.74 kWh of thermal energy, the generator is about 24.5 percent efficient at converting the thermal energy of the fuel to electrical energy. (10/40.74 = 0.245)
• Kilowatt-hour (kWh) - A measure of electrical energy where one kWh is equal to 1 kilowatt (1,000 watts) of power expended for 1 hour.
• Power - The rate at which energy is converted from one form to another, such as from the thermal energy of a fuel to the kinetic energy of a steam turbine to the electrical energy from the generator. Electrical power is typically measured in watts and mechanical power is typically measured in horsepower. One horsepower is equivalent to 33,000 foot-pounds of force per minute and one watt is equivalent to one joule per second.
• Quad - One quadrillion (1 X 10¹⁵) BTUs. This unit is typically used to quantify energy consumption at a national scale because the numbers are so large. As an example, the energy content of all the coal consumed in the U.S. in 2005 was 22.9 quads.

Conversion Factors

Source (including definitions): Energy Information Administration
1 - Values shown are the theoretical maximum heat content, which assumes recovery of the latent heat of vaporization in the water vapor produced in combustion. The estimated lower values would yield a similar ratio between gasoline and ethanol.