Remember that energy is the “ability or capacity to do work,” where “work” is defined as a change in energy. Energy is measured in a variety of units including calories, kilowatt hours (kWh), British Thermal Units (Btu). Power is measured in watts (or joules per second) and is the rate at which energy is generated or used.
Fuels are converted into useful energy according to their heat content. The heat content of a fuel is normally measured in Btus. One Btu is the amount of energy required to raise the temperature of one pound of water (about a pint) one degree Fahrenheit. The relative efficiency of a fuel or technology is summarized in the “heat rate” – the amount of fuel required to generate one kilowatt-hour (kWh). As might be expected, not all fuels are created equal. Comparison of fuels is possible by converting them into useful energy according to their heat content as measured in Btus.
Equation 1: Heat rate = Btu/kWh
Under ideal conditions, in which the energy input is equal to the energy output, the heat rate is equal to 3,412 Btu/kWh. But not all energy-conversion processes are ideally efficient. So Equation 1 must be modified to account for these inefficiencies.
Equation 2: Heat rate = (ideal heat rate)/efficiency = (3412 Btu/kWh)/efficiency
Table 1 summarizes the average heat content of several familiar fuels. These numbers are averages because variations in the quality can affect the efficiency with which these fuels can be converted into useful energy.
|Fuel||Amount||Heat Content (Btus)|
|Oil||1 barrel*||5,800,000 (138,095/gal)|
|Natural gas||1 cubic foot||1,026|
|Diesel or heating oil||1 gallon||139,000|
|White oak (20% moisture content) fn||1 cord||27,000,000|
* 1 barrel of oil = 42 gallons.
** Heat content of coal is an average of bituminous coals. Lignite (brown coal) will have a lower heat content. Anthracite will have a higher heat content.
*** The number for electricity assumes perfectly efficient conversion of fuel into electricity.