The standard unit of energy in the International System of Units (SI) is the joule (J), equal to one watt second. One kilowatt hour is 3.6 megajoules.
The kilowatt hour is standard unit of measurement for electrical energy delivered to consumers by electric retailers.
A "British thermal unit" (Btu) is a measure of the heat content of fuels. It is the quantity of heat required to raise the temperature of 1 pound of liquid water by 1°F at the temperature that water has its greatest density (approximately 39°F).
One practical way to compare different fuels is to convert physical units of measure (such as weight or volume) into a common unit of measurement based on the energy content of each fuel. The British thermal unit (Btu) is a widely used measure of energy content. Popular physical units of measurement that are converted to Btu are; barrels of oil equivalent, metric tons of oil equivalent, and metric tons of coal equivalent.
It takes between 7,000 to 11,500 Btu of primary fuel to generate a kilowatt hour in a fossil fuel fired power plants. The number of BTUs of heat required to produce a kilowatt-hour of energy in a power plant is referred to as the heat rate of the power plant. For more information on heat rates see this report: