Absorber   The plate in a solar hot water collector or pool collector that converts solar radiation to heat energy. Generally matte black or selective surfaces on copper.

Absorptance   The ratio of energy absorbed by a surface relative to the energy striking it. Thermal collectors have high absorptive coatings resulting in absorptance of 0.95 to 0.98

AC Disconnect   One of the balance of system components, this utility serviceable disconnect switch allows the current being produced from the photovoltaic system to be isolated from the utility feed during times of maintenance, service or repair.

Alternating Current (AC)   The type of current that most utility interconnected homes and commercial structures run on. The direction of this type of current reverses at regular intervals usually 100/120-reversals or 50/60-cycles per minute making it easy for the utility providers to control.

Ambient Temperature   Temperature of the surrounding outside air

Ampere (A)   A unit of electrical current which measures the flow of electrons (similar to the measure of flow rate of water through a hose)

Amp-hour Capacity (Ah/AM)   Referring to the size of a battery, this is a measurement of the flow of current per hour.

Angle Of Incidence   The angle at which solar radiation strikes a surface (collector)

Array   A group of thermal collectors or photovoltaic modules organized to produce a desired amount of energy

Balance of System Components   All components of a solar system other than the actual array (includes everything from design costs through installation and service/maintenance)

Battery Back-up   A photovoltaic system designed with batteries of a specified amp-hour capacity to provide current to a residential or commercial structure or its key loads for a predetermined period of time

British Thermal Unit (BTU)   The quantity of heat needed to raise 1 pound of water by 1° Fahrenheit. 8¾ BTU’s will raise temperature of 1 gallon of water 1°F

Charge Controller   Utilized only in systems which incorporate batteries, this component controls the flow of current to and from the batteries to protect them from over-charging and over-discharging

Collector (Solar Collector)   Device that collects solar radiation and converts it to heat

Collector Efficiency   The radio of usable heat energy extracted from a collector to the solar energy striking the cover

Collector, Evacuated Tube   A collector which consists of parallel rows of glass tubes connected to a header pipe. Each tube has the air removed from it to eliminate heat loss through convection and radiation.

Collector, Flat-Plate   A flat, rectangular, insulated, weatherproof box containing an absorber plate under a cover plate.

Conduction   heat flow in any direction due to temperature variations within a material

Convection   The motion of a fluid by which heat may be transported. See also ‘Thermosyphoning’, ‘Conduction’, and ‘Radiation’

Cover Plate   Also referred to as glazing, usually a sheet of tempered glass or transparent plastic placed above the absorber in a flat plate collector.

Current   the flow of electrical energy measured in amperes (A)

DC Disconnect   a disconnect switch which isolates the photovoltaic array from the other system components during times of maintenance/service.

DHW (SDHW)   Solar domestic hot water

Direct Current (DC)   A type of electricity which flows through a conductor in only one direction with low voltage and high current. Most standard appliances will not work on DC, so the electricity must be inverted to AC.

Emittance   A measure of the propensity of material to radiate energy to its surroundings. Solar collectors have low emissivity coatings.

FSEC   Florida Solar Energy Center, organization that provides research, testing, and certification of solar thermal collectors and systems

Galvanic Corrosion   Condition caused as a result of a conducting liquid making contact with two different metals which are not properly isolated.

Grid-Tied   The photovoltaic system that is connected to the utility grid allowing the system and the grid to interact (allows for net-metering).

Header   Tubes running horizontally across the bottom and top of absorber plates. See also ‘Risers’

Heat Exchanger   A device, such as a copper coil, immersed in a tank or tube of water, used to transfer heat from one fluid to another

Heat Exchanger, Double Wall   Heat exchangers using a non-potable (antifreeze) heat transfer fluid that is separated from the potable water by two walls and a small gap, thus allowing a leak of heat transfer fluid to become visible rather than enter the potable water. These heat exchangers are required by most plumbing codes.

Hybrid System   Often referring to solar pool heating, is a system which uses solar collectors in addition to a conventional heater.

Insolation   The total amount of solar radiation – direct, diffused, and reflected – striking a surface exposed to the sky.

Inverter   The component of a photovoltaic system which changes DC to AC (essential in both grid-tied and stand-alone systems).

Junction Box (PV)   An enclosure on the back of a module where photovoltaic modules are connected to form a string.

Junction Box (roof)   An enclosure mounted near the array where strings come together to be run to the inverter.

Kilowatt (kW)   A unit of electrical power; equals 1000-watts

Kilowatt-hour (kWh)   A unit of energy; equals 1000-watts acting over one hour.

Magnetic South   The direction on a compass pointing due-South, away from the North Pole.

Megawatt (MW)   A unit of electrical power; equals 1000-kilowatts

Module   A collection of photovoltaic cells connected together, as well as other components (i.e. junction box, frame, wires, etc.), designed to produce a specified number of watts under standard test conditions (STC). Also referred to as a solar panel.

National Electric Code (NEC)   The guidelines for all types of electrical installations.

Net-metering   An agreement with the utility provider which allows grid-tied systems to back-feed the grid during times when production exceeds usage. During these times the utility meter actually runs backwards, creating a “debit and credit” situation between the producer and the utility provider.

Orientation   The direction that a solar array faces with regard to magnetic directions (N, E, S, W).

Passive System   A system which converts solar energy into usable thermal energy without using mechanical power. See ‘thermosyphon’.

Photovoltaics (PV)   The conversion of light into electricity.

Racking (rails)   The components used to secure a domestic hot water, pool, or photovoltaic system in place; can be ground, deck, roof or pole mounted depending on the application and the orientation of available space.

Radiation   The flow of energy across open space via electromagnetic waves, such as visible light.

Resistance (R)   A unit measured in ohms, it is the opposition to the flow of electrons produced by a conductor.

Risers   Tubes that run vertically, inside the absorber plate, between the headers.

Seasonal Efficiency   The ratio, over an entire heating season, of solar energy collected and used against the solar energy striking the array

Selective Surface   A surface used for absorber plates that absorbs radiation of one wavelength (i.e. sunlight) but emits little radiation of another wavelength (i.e. heat)

Solar Access   The amount of sunlight a photovoltaic array should receive based upon orientation, obstructions and average annual sun hours.

Solar Cell   The smallest element in a solar module, it is directly responsible for the conversion of light into electricity. Usually made of mono- or poly-crystalline or amorphous silicone.

Solar Index   The ratio of measured solar radiation to an expected number if the sky is clear, with an average clear sky day being 100%

Solar Noon   The time the sun crosses the meridian of the site, this is the highest point the sun gets in a day. Also referred to as Solar South

Solar South   See ‘Solar Noon’

SRCC   Solar Rating and Certification Committee. Organization that rates collector and system performance.

Stagnation   In a closed loop glycol system, when the pump does not operate as intended and fluid sits in the collectors under full sunshine. The fluid may be damaged from extreme high temperatures and a pressure relief valve may open.

Standard Test Conditions (STC)   Conditions under which a module is tested in a laboratory; each manufacturer has slightly different STC’s, but most center around the following criteria: 25 degrees Celsius, 1.5 atmospheres of air pressure and 1000-watts per square meter of light energy.

Standby Heat Loss   Heat lost through storage and piping, even when not hot water is used.

Thermistor   an electric device which changes its resistance based upon temperature.

Thermosyphon   A process which makes water circulate automatically from a warm collector or heat exchanger to cooler water above it.

Tilt Angle   the angle a collector or module surface forms with the horizon.

Volts (V)   the unit that measures electrical force (the force required for one amp to flow through a resistance of one ohm).

Watt (W)   The basic unit of electrical power.