Glossary of Solar Terms

Glossary of Solar Terms

Confused by some of the terms and acronyms used in the solar industry? You’re not alone. To help, we’ve compiled this glossary of solar terms.


– a tubular device that fluid passes through, absorbing the solar radiation and converting it to thermal energy to be used for other devices.


– The rate of decrease in the intensity of solar energy as it passes through the absorber.

Active System

– A solar heating or cooling system that collects solar radiation and transfers the energy to a storage system to distribute the heat.

Alternating currents

– Electric currents with reversed direction of flow at frequent intervals.

Auxiliary Heat

– A backup conventional heating system used during periods of cloudiness or intense cold when a solar heating system does not provide enough heat.


– The angle distance between true south and the point on the horizon directly below the sun.

British Thermal Unit (BTU)

– The amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit.


– The amount of heat energy needed to raise the temperature of one gram of water one degree Celsius.

Coefficient of Heat Transmission

– The rate of heat loss, in BTU per hour, through a square foot of material when the difference between indoor and outdoor air temperatures is one degree Fahrenheit – the lower the number, the greater the insulating characteristics of the material.


– A device that collects, absorbs and transfers solar radiation and converts it to heat.

Collector Efficiency

– The ratio of usable heat energy captured by a collector compared to the solar energy available.


– The transfer of heat due to temperature variations within a material or two materials that are in direct physical contact.

Degree Day

– A unit for measuring how much the outdoor temperature deviates for some fixed reference point (usually 65°F.). One degree day is counted for each degree below or above the reference point.

Design Heat Load

– The amount of heat to be supplied by a system. It is determined by the estimated maximum heat loss from a house under the most severe winter conditions.

Design Temperature

– The temperature that a system must sustain under the most sever conditions.

Diffuse Radiation

– Indirect sunlight that is scattered from air molecules, water vapor and dust. Diffuse radiation can be used by a flat-plate collector.

Direct Radiation

– Solar radiation that comes straight from the sun. Direct radiation can be used by both a flat-plate collector and a concentrator system.

Drain down System

– Provides freeze protection with valves that open and dump the water in the system to drain at a preset low temperature. Collectors and piping must be pitched or sloped so that the system can drain down using gravity.

Drain back System

– A solar system that automatically drains heath transfer fluid into a tank by gravity. Drain back systems are available in one or two tank configurations. A heat exchanger is necessary, because the city inlet pressure would prevent draining.

Flat Plate Collector

– A solar collection device in which direct solar radiation is absorbed by the absorber plates, transferred to a fluid that circulates through the collector and is converted into heat.

Heat-transfer fluid

– A gas or liquid used to move solar energy.


– The pipe that runs across the edge of multiple solar collectors, gathering or distributing the heat transfer fluid to risers or collectors. This insures that equal flow rates and pressure are maintained.

Heat Capacity

– A property of a material denoting its ability to absorb heat.

Heat Exchanger

– A device, such as a coiled copper tube immersed in a tank of water, used to transfer heat from one fluid to another when they are physically separated.

Heat Storage

– A device or medium that absorbs solar heat and stores it for later use.

Heat Pump

– A mechanical device extracts available heat from area and transfers it to another, thereby cooling the first and warming the second. Because heat pumps move the heat rather than generating it they can provide up to four times the amount of energy they consume.

Hybrid Solar Energy System

– A system that uses both active and passive methods in its operation.

Indirect System

– A solar heating or cooling system in which the solar heat is collected exterior to the building and transferred inside using ducts or piping and, usually fans or ducts.

Infrared Radiation

– Electro-magnetic radiation from the sun that has wavelengths slightly longer than visible light.


– The total amount of solar radiation: direct, diffused and reflected, striking a surface.


– A solar device that converts direct current electricity from a solar module to alternating current in order to supply power to an electricity grid.


– A measure of solar radiation; equal to one calorie per square centimeter.

Liquid Type Collector

– A solar collector using liquid as the heat transfer flu

Net-metering grid-tie

– The state rule that allows utility customers to back-feed their electric meters. Therefore, if your system is generating more power than you are consuming you can ‘sell’ electricity back

Open System

– A system where matter or energy can flow into and/or out of the system, in contrast to a closed system, where energy can enter or leave but matter may not.

Passive System

– A solar heating or cooling system that uses structural elements of a building instead of external mechanical power to move the collected solar heat.

Photovoltaics (PV)

– The technology of converting solar energy into electricity.

Photovoltaic Cells

– Also called a solar cell, it is made up of a variety of semiconductor devices that convert solar energy into electricity.


– An instrument for measuring solar radiation or global irradiance.

Radiant Panels

– Panels with integral passages for the flow of warm fluids, either air or liquids. Heat from the fluid is conducted through the metal and transferred to the rooms by thermal radiation.


– The transfer of heat through open space via electromagnetic waves, such as visible light.

Resistance, or R Value

– A measure of the ability of a material to retard the flow of heat. The higher the R-value the greater its insulating properties.

Selective Surface

– A coating applied to surfaces as a means of increasing operation temperature and/or efficiency. The selectivity is defined as the ratio of radiation-absorption to radiation emission. Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface.

Solar Radiation

– Electromagnetic radiation emitted by the sun.

Specific Heat

– The amount of heat, in BTU, required to raise the temperature of one pound of a material 1°F.

Sun Path Diagram

– A circular projection of the sky vault. Similar to a map, it is used to determine solar positions and calculate shading.

Thermal Mass

– The ability of a building or material to absorb heat. High-density materials that remain at the same temperature or fluctuate very slowly are said to have a high thermal mass.

Tilt Angle

– The angle that a flat-plate collector surface is set to face the sun relative to a horizontal plane.