Energy, in its forms as work, heat, and electric power, provides such services as powering a car or providing light. It can come from renewable sources, such as the sun or wind; from chemical reactions, such as combustion of fossil or renewable fuels; or from nuclear reactions. Energy is usually measured in British thermal units (Btu) or kilowatt-hours (kWh) (or joules; 1 watt-hour is equivalent to 3600 joules). Delivered energy is a measure at the point-of-use (site) of the amount of energy delivered to a consumer without adjusting for the energy lost in transforming a fuel or other form of energy to electricity, transmitting the energy to the point where it will be used, and then distributing it to individual users. Primary energy is the amount of delivered energy adjusted upward to account for the energy that is lost in the transformation and delivery of that energy to an end user, such as a residential housing unit.
Energy intensity is a measure of the amount of energy used per unit of output for a company, industry, or the whole economy. For example, the energy intensity of steel production represents the amount of energy used to produce a ton of steel, or Btu per ton. Energy intensity can also represent the amount of energy used per dollar of output—or, for the whole economy, per dollar of gross domestic product (GDP). For example, the energy intensity of the U.S. economy is about 9000 Btu per dollar of GDP, equivalent to a bit less than one pound of coal or a bit more than one cup of gasoline per dollar.
Energy efficiency is a measure describing how much useful work can be obtained from a system from a given amount of input energy. (A more formal definition derived from thermodynamics is given in Appendix E.) This report deals primarily with technologies for realigning improvements in energy efficiency, which the panel defines as accomplishing a specified objective using less energy. The objective might be to heat a room to a certain temperature, or provide a certain amount of light. For example, a compact fluorescent lamp is more efficient than an incandescent lamp, in that it provides the same number of lumens and quality of light as an incandescent lamp for only one-quarter to one-third of the energy input. Energy efficiency can be expressed directly as a dimensionless ratio—in this case, the ratio of the energy fed into a lightbulb to that which is radiated as light.
Conservation is usually understood to mean action taken to reduce the amount of energy used by changing behavior, such as turning off personal computers when not in use or setting back the thermostat in winter. It can also involve using technologies, such as room occupancy sensors for lighting, which reduce energy use without someone having to remember to turn off the light.