Common energy terms explained in plain language.
Uncontrolled air leaking into or out of a building through gaps, cracks, and openings. Infiltration wastes energy because conditioned air escapes and must be replaced. See our draft sealing guide.
A unit of energy commonly used to describe heating and cooling capacity. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. Higher BTU ratings mean more heating or cooling power.
A type of light bulb that uses less energy than incandescent bulbs. CFLs have largely been replaced by LED bulbs, which are even more efficient and don't contain mercury.
Heat transfer through direct contact—for example, heat passing through a window pane. Materials that conduct heat easily (like glass or metal) allow more heat to pass through than good insulators.
Heat transfer through air movement. Warm air rises and cool air falls, creating natural circulation. Fans use forced convection to move air and distribute heating or cooling.
A noticeable flow of air, usually where it shouldn't be—like around a window or under a door. Drafts indicate air leaks that should be sealed for better efficiency.
Getting the same output (light, heat, cooling, work) with less energy input. An efficient appliance does the same job as a less efficient one while using less electricity or fuel.
A standardized label on appliances showing their energy efficiency compared to similar products. Labels vary by country but generally help consumers compare efficiency when shopping.
Heat entering a building from outside—through windows, walls, roof, or air leaks. In summer, reducing heat gain reduces the need for cooling. Solar heat gain through windows is often the biggest source.
Heat escaping from a building to the outside—through windows, walls, roof, or air leaks. In winter, reducing heat loss reduces heating needs.
A system that moves heat rather than generating it. Heat pumps can both heat and cool, and are typically more efficient than traditional heating systems. Heat pump dryers are significantly more efficient than conventional dryers.
Heating, Ventilation, and Air Conditioning—the systems that control temperature and air quality in buildings.
Material that resists heat transfer. Insulation in walls, attics, and floors reduces heat loss in winter and heat gain in summer. Better insulation means less energy needed for heating and cooling.
A unit of energy commonly used in electricity billing. One kWh equals using 1,000 watts for one hour (for example, running a 100-watt bulb for 10 hours).
A highly efficient lighting technology that uses a fraction of the energy of incandescent bulbs and lasts much longer. Modern LEDs produce warm, pleasant light quality. See our lighting guide.
In energy terms, the amount of power being used at any given time. A "base load" is the minimum constant power use; peak loads occur during high-demand periods.
A measure of light output (brightness). When buying bulbs, lumens tell you how much light you'll get. Higher lumens = brighter light. This is more useful than watts when comparing efficient bulbs.
Energy used by devices when they're switched "off" but still plugged in. Many electronics draw power continuously to maintain clocks, remote control readiness, or quick startup. See our phantom loads guide.
A measure of thermal resistance—how well a material resists heat transfer. Higher R-values indicate better insulation. R-values are often used to rate insulation materials and windows.
Heat transfer through electromagnetic waves (like infrared). The sun's heat reaches us through radiation. Low-e window coatings reduce radiant heat transfer.
A measure of air conditioner efficiency—higher SEER ratings mean more efficient cooling. When shopping for AC units, compare SEER ratings for efficiency.
Lowering (or raising, for cooling) thermostat settings during periods when full comfort isn't needed—like when sleeping or away from home. Setbacks reduce energy use.
Heat entering a building through sunlight, especially through windows. Desirable in winter for free heating; problematic in summer when it increases cooling needs.
Areas where heat transfers more easily through the building envelope—often at structural elements like studs or around window frames. Thermal bridges are weak points in insulation.
A device that monitors temperature and controls heating or cooling systems to maintain a set temperature. Programmable and smart thermostats can automate temperature setbacks.
A measure of how well a material conducts heat—the opposite of R-value. Lower U-values indicate better insulation. Often used for windows.
A unit of power—the rate at which energy is used. A 100-watt light bulb uses energy at a rate of 100 watts. Lower wattage for the same output means more efficiency.
Protecting a building from weather—sealing drafts, adding insulation, improving windows. Weatherization reduces both heat loss and heat gain.
Material applied around doors and windows to seal gaps and prevent air leaks. Available in various forms including foam tape, V-strip, and door sweeps. See our draft sealing guide.