Alternating current (AC)
Electric current in which the direction of the flow is reversed at frequent intervals, 100 times per second in Europe (50 cycles per second) and 120 times per second in North America. Opposite of AC is direct current (DC).

Ampere-hour (Ah)
A current of one ampere running for one hour.

Autonomous system
A stand-alone photovoltaic (PV) system that has no backup generating source and relies only on solar energy to meet the needs of the load. May or may not include storage batteries.

Balance of system
The parts of a PV system other than the PV array, power conditioning equipment and batteries. This may include switches, controls, meters, trackers and a supporting structure for the PV array.

Battery
For off-grid systems a battery is used to provide energy storage. Nearly all batteries used for PV systems are of the lead-acid type (with a small quantity of antimony to reduce self-discharge). All PV and backup power batteries are deep-cycle i.e. designed to be discharged down to 50% or more without damage so that they can supply power over a long period of time (in contrast to a car battery, for example, which is usually only discharged down to 3 to 5 %). The lifetime of a battery varies depending on factors such as how it is used, how it is maintained and charged, and temperature, but is typically between 5 and 10 years.

Charge controller
Charge controllers are typically used in off-grid photovoltaic power systems. The primary function of a charge controller (or regulator) is to maintain the battery at the highest possible State of Charge (SOC) and provide the user with the required quantity of electricity, while protecting the battery from deep discharge (by the loads) or extended overcharge (by the PV array). Most charge controllers operate via voltage regulation set points. However, as voltage is not representative of the true SOC, new algorithms are being developed to evaluate the state of the battery, based on Ah or combined VAh monitoring. Additional features such as battery temperature or wire compensation, meters and alarms can enhance the ability of the charge controller to meet the load demand and extend battery lifetime. Other functions such as MPPT, DC/DC conversion, anti-theft protection, load management, pre-payment and data logging can also now be built into the charge controller.

Direct current (DC)
Electric current in which electrons are flowing in one direction only. Opposite of alternating current (AC).

Easement
An oral or written legal agreement defining an interest in exclusive, common or bipartisan use of private property or air/space above that property. A common form of easement is the concept of “right of way,” as when an electric utility has the right of way to extend electrical transmission lines across private property. See also “Restrictive covenant.”

Energy payback time
The time required for any energy producing system or device to produce as much useful energy as was consumed in its manufacture and construction. For PV the energy payback times is 2 to 4 years.

Grid
Network of transmission lines, substations, distribution lines and transformers used by central power systems

Grid-connected distributed photovoltaic power system
System installed on consumers' premises usually on the demand side of the electricity meter. This includes grid-connected domestic photovoltaic power systems and other grid-connected photovoltaic power systems on commercial buildings, motorway sound barriers, etc. These may be used for support of the utility distribution grid.

Horsepower (hp)
An imperial system unit of power equivalent to 746 W.

Hybrid PV system
A PV system that includes other sources of electricity generation, such as a wind or diesel generator.

Installed power
Power delivered by a photovoltaic module or a photovoltaic array, under standard test conditions (irradiance of 1 000 W/m2, cell junction temperature of 25°C, AM 1, 5 solar spectrum). Also said STC output power. Unit: Watt

Inverter
Device that converts direct current (DC) into alternating current (AC).

I-V curve
A graphical presentation of the current versus the voltage from a photovoltaic cell as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance.

Kilowatt (kW)
One thousand watts.

v Kilowatt-hour (kWh)
One thousand watt-hours, unit of energy (power expressed in kW multiplied by time expressed in hours)

Load
Anything in an electrical circuit which, when the circuit is turned on, draws power from that circuit (lights, appliances, tools, pumps, etc.).

Lumen
A metric measurement of the rate at which light is emitted from a source.

v Maximum power point tracker (MPPT)
Charge controller that continuously tracks the maximum power point (MPP) of a PV module or array, thus increasing its efficiency. The MPP is the point on a current-voltage (I-V) curve where a PV device produces maximum power.

Mounting structure
With the rapid growth of grid-connected distributed systems, a wide range of products have been developed for installing PV modules on buildings. These include mounting structures for PV facades, roof profiles, flat roofs and even 'PV tiles' that can be used to replace conventional roof tiles. New products are addressing the need for ease of integration into the building envelope and aesthetic appeal.

Off-grid domestic photovoltaic power system
System installed in households and villages that are not connected to the utility grid. Usually, a means to store electricity is used (most commonly lead-acid battery). Also said "stand-alone photovoltaic power system".

Off-grid non-domestic photovoltaic power system
System used for a variety of applications such as water pumping, remote communications, telecommunication relays, safety and protection devices, etc. which are not connected to the utility grid. Usually a means to store electricity is used. Also said "stand-alone photovoltaic power system".

Open-circuit voltage
The voltage across a PV cell in full sunlight when there is no current flowing; the highest possible voltage.

Parallel connection
A method of interconnecting two or more devices that generate or use electricity, such that the voltage produced, or required, is not increased, but the current is the sum of the two. Opposite of “series connection” (see entry).

Peak power
PV modules are rated by their total power output. The peak power is the amount of power output a PV modules produces at Standard test conditions (STC) of a module operating temperature of 25 degrees Celsius in full sunshine (irradiance) of 1 000 watts per square meter. This is a clear summer day with sun approximately overhead and the cells faced directly towards the sun. Unit: Watt (W), also written Wp by professionals in the field.

Performance ratio
Ratio of the final annual (monthly, daily) yield to the reference annual (monthly, daily) yield, where the reference annual (monthly, daily) yield is the theoretically annual (monthly, daily) available energy per kilowatt of installed power.

Photovoltaic (PV) array
An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure with a common support or mounting. In smaller systems, an array can consist of two modules plus a support structure or mounting.

Photovoltaic (PV) cell
A device that converts light directly into electricity. The building block of a PV module.

Photovoltaic (PV) module
A number of PV cells electrically interconnected (in either series or parallel) and mounted together, usually in a sealed unit of convenient size to make shipping, handling and assembly into arrays easier.

Photovoltaic panel
A group of modules fastened together, pre-assembled and wired, designed to serve as an installable unit in a photovoltaic array.

Photovoltaic (PV) system
A complete set of components for converting sunlight into electricity by the PV process, including the array and balance of system components.

Photovoltaics
The process that produces electricity from sunlight. "Photo" refers to light and "voltaic" to voltage.

PV
Abbreviation of photovoltaic (adjective) or photovoltaics (noun).

v PVPS
Abbreviation of photovoltaic power system(s).

Power-conditioning equipment
Electrical equipment used to convert power from a PV array into a form suitable for subsequent use. A collective term for inverter, charger, battery charge controller and blocking diode.

Restrictive covenant
A specialized type of easement that can be used to protect access to sunlight or wind flow for solar or wind energy applications. See also “Easement.”

Series connection
A method of interconnecting devices that generate or use electricity so that the voltage, but not the current, is additive. Opposite of “parallel connection” (see entry).

Short circuit current
The current flowing freely from a PV cell through an external circuit that has no load or resistance; the highest current possible.

Stand-alone PV system
A photovoltaic system not connected to a main electric grid. May be solar-only or hybrid. May or may not have storage batteries, but most stand-alone systems require batteries or some other form of storage (e.g. water reservoir for pumping).

Stand-off mounting
Technique for mounting a PV array on a sloped roof that involves mounting the modules a short distance above the pitched roof and tilting them to the best angle.

Standard test conditions (STC)
The testing conditions to measure photovoltaic cells or modules nominal output power. Irradiance level is 1 000 W/m2, with the reference air mass 1,5 solar spectral irradiance distribution and cell or module junction temperature of 25°C.

Telemetry
The remote measurement of any physical quantity using instruments that convert the measurement into a transmittable signal.

Turnkey price
Price of an installed photovoltaic system excluding VAT/TVA/sales taxes, operation and maintenance costs but including installation costs. For an off-grid photovoltaic system, the prices associated with storage battery maintenance/replacement are excluded. If additional costs are incurred for reasons not directly related to the photovoltaic system, these are excluded. For example if extra costs are incurred fitting PV modules to a factory roof because special precautions are required to avoid disrupting production, these extra costs are not included. Equally the additional transport costs of installing a telecommunication system in a remote area are not included.

Watt (W)
SI unit of power. Symbol is W. When written as Wp (peak watt) also means under standard test conditions (STC). Multiples like kW (one thousand watts = 1 000 W) or MW (one million watts = 1 000 000 W) are also used.

Watt-hour (Wh)
A quantity of energy. One watt-hour of electricity is consumed when one watt of power is used for one hour.