Glossary of Solar Terms

Alternating Current (AC)
Electric current that reverses direction, usually many times per second. Opposite of direct current (DC). Most electrical generators produce alternating current.

Ballasted System
A method in which a solar PV system is secured on a rooftop using weight. No penetration of the roof is required.

Crystalline Modules
High output solar PV modules manufactured using silicon wafers.

Direct Current (DC)
Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges always flow in the same direction, which distinguishes it from alternating current (AC).

Kilowatt (kW)
A unit of electrical power equal to 1,000 watts, which constitutes the basic unit of electrical demand. The watt is a metric measurement of power (not energy) and is the rate (not the duration over which) electricity is used. 1,000 kW is equal to 1 megawatt (MW).

Kilowatt Hour (kWh)
The use of 1,000 watts of electricity for one full hour. Unlike kW, kWh is a measure of energy, not power, and is the unit on which the price of electrical energy is based. Electricity rates are most commonly expressed in cents per kilowatt hour.

Megawatt (MW)
Unit of electrical power equal to one million watts; also equals 1,000 kW.

Net Metering Agreement
An agreement with the local utility which allows customers to reduce their electric bill by exchanging surplus electricity generated by the solar PV system. Under net metering, the electric meter runs backwards as the customer-generator feeds extra electricity back to the utility.

Renewable Energy Certificate (REC)
A REC represents the renewable attributes of energy generated by a solar facility. In most markets, a REC equals 1 MWh of energy generated. Some utilities will purchase these RECs from a system owner in order to meet their Renewable Portfolio Standards.

Roof Penetrating
A method in which a solar PV system is secured on a rooftop by physically attaching the system to the rooftop. Penetrations are then sealed to prevent leaks.

Thin-Film Laminate
A flexible solar PV module that is adhered directly to the rooftop.

Thin-Film Modules
Solar PV modules manufactured using CdTe or other materials. Typically lighter and less efficient than crystalline panels.