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A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.


A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate.

ampere (amp) —

A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

ampere-hour (Ah/AH) 

A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

AMP Load 

AMP load refers to the amount of demand an electrical circuit puts on a power source, such as a battery or the circuit breaker between the electrical item and the power line that runs to a house or building. An AMP, short for ampere, represents a unit of measure for the amount of electrical current charge required by an electrical appliance per second of use. In most cases, this term is in direct reference to battery-powered appliances and the amount of time the battery will power the device during its use. Batteries are rated as being capable to handle a specific range of ampere usage, or AMP load, by how long they will power the item if the item requires a 1 AMP load to power itself effectively.

For instance, a battery with an ampere-hour rating of 16 Ah will power a device requiring a 2 AMP load for eight hours before needing recharging or replacing; likewise, if the device requires a 1 AMP load, then the battery will power the device for 16 hours before the battery will not be able to effectively power the device. Rechargeable batteries are usually much more efficient than replaceable batteries, especially in devices that require more than a 2 AMP load. The actual AMP load required by the device depends on the function of the device itself and how much power the device requires at a constant rate in order to function effectively.

antireflection coating 

A thin coating of a material applied to a solar cell surface reduces the light reflection and increases light transmission.

azimuth angle 

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


balance of system

Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.


Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system

battery available capacity 

The total maximum charge, expressed in ampere-hours, can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, the initial state of charge, age, and cut-off voltage.

battery capacity 

The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

battery cell 

The simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for all the above.

battery cycle life 

The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

battery energy capacity 

The total energy available, expressed in watt-hours (kilowatt-hours), can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

battery energy storage 

Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

battery life 

The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

bypass diode 

A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.


cadmium (Cd) 

A chemical element used in making certain types of solar cells and batteries.

charge controller 

A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system's operational status.

charge factor 

A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate.

charge rate 

The current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.

cloud enhancement 

The increase in solar intensity caused by reflected irradiance from nearby clouds.

combined collector 

A photovoltaic device or module that provides useful heat energy in addition to electricity.

concentrating photovoltaics (CPV) 

A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells.

concentrating solar power (CSP) 

A solar technology that uses mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator.


A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.


The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.


A unit that converts a direct current (dc) voltage to another dc voltage.

crystalline silicon 

A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

current at maximum power (Imp) 

The current at which maximum power is available from a module.

cutoff voltage 

The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.


The discharge and subsequent charge of a battery.


days of storage 

The number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability.