Megawatts, kilowatts – what’s the difference? We understand that talking about energy can be confusing. To help you learn about commonly used energy terms, read our energy glossary. If you can’t find the term you’re looking for, refer to the Department of Energy Glossary.
Heating, Cooling & Ventilation
BTU
BTU, which stands for British Thermal Unit, is a measurement of heat energy. One BTU is the amount of heat required to raise the temperature of one pound of water by 1°F. For example, the heat produced by burning one wooden kitchen match is the equivalent of one BTU.
Heat pump
A heat pump is like an air conditioner with a reversible valve, which allows it to operate in reverse, taking heat from your house and pushing it outdoors in the summer, and pulling heat from outdoor air and bringing it back into your house in the winter. Heat pumps are more efficient than other forms of heating because they do not actually create heat; they just move it from one location to another.
EER
The instantaneous measurement of the cooling efficiency of your air conditioner or heat pump is known as its Energy Efficiency Ratio, or EER. An air conditioner or heat pump with a high EER is very efficient.
Insulation
Material that resists the transfer of heat, usually by incorporating small pockets of air. Insulation is rated in terms of thermal resistance, called R-value, which shows the resistance to heat flow. If the R-value is high, then the insulating effectiveness is high as well. The R-value of thermal insulation depends on the type of material, as well as its density and thickness.
Window Efficiency
Thermal break
A substance used in the main body of the window frame that does not transmit heat well, like plastic. Thermal breaks reduce the heat being transferred through the frame and can also be used in multi-pane windows as part of the spacer between panes of glass.
U-Value
A measurement of heat flow. The slower the window transfers heat in and out of a house, the lower the U-value.
R-Value
A way to measure heat resistance. The higher the R-value of a house, the better the window resists the transfer of heat; this is the inverse of the U-value.
Water Heater Efficiency
Peak hour demand
The maximum water usage allowed, in gallons per hour, during the time of day when your family is likely to use the most hot water.
Energy factor
Also known as EF, energy factor measures the efficiency of a water heater’s storage, which is based off an average hot water usage of 64 gallons per day. The more efficient the water heater is, the higher the EF.
Water heater temperature
The temperature of your water heater is controlled by a thermostat. Money can be saved by lowering the temperature of the water heater to 120°F (about midway between the "low" and "medium" settings). If your dishwasher doesn't have a heating element, leave your thermostat on the "medium" setting (140°F).
Lighting Efficiency
Lumen
The total amount of light given off from a source is called a lumen. Lumens are used to measure or rate the output of lamps.
Lamp
Used to refer to the source of light or the light bulb itself, not the fixture that contains the light bulb.
Incandescent lamp
Glass light bulbs with a filament inside, or the most common source of light. Around 90% of the energy used by an incandescent lamp is given off as heat and not light.
Compact fluorescent lamp (also known as a CFL)
This lamp can replace standard incandescent lights and is a smaller, more compact version. Contained within the light is a gas-filled tube and a electronic or magnetic ballast. Compact Fluorescent Light Bulbs (CFLs) contain small amounts of mercury, please dispose of properly.
