Off-Grid Solar Glossary: PV, Battery & Inverter Terms

A sealed VRLA lead-acid battery in which a glass-fibre mat holds the electrolyte. AGM batteries are maintenance-free in normal use and can deliver relatively high currents, but they still require the correct charging profile.

A charging stage used mainly with lead-acid batteries after bulk charging. The charger holds a controlled voltage while the charging current gradually falls as the battery approaches full charge.

Electric current that periodically reverses direction. Household and grid-connected equipment commonly operates on AC, for example 230 V AC at 50 Hz in much of Europe.

A non-crystalline semiconductor material, often used in thin-film photovoltaic modules.

The ampere (A) is the SI base unit of electric current. It is defined by fixing the numerical value of the elementary charge at exactly 1.602 176 634 × 10⁻¹⁹ coulomb, where one coulomb equals one ampere-second.

The unit of electric current. Current indicates how much electric charge is flowing through a circuit.

A unit of electric charge commonly used to state nominal battery capacity. Ampere-hours alone do not describe stored energy unless the battery voltage is also known; nominal energy is approximately volts × ampere-hours = watt-hours.

Alliance for Rural Electrification.

Average Revenue per user.

One or more electrically connected photovoltaic modules operating as a single solar-power source.

The compass direction a solar surface faces. In the northern hemisphere, an azimuth close to south normally provides the highest annual yield when shading and site conditions are favourable.

All parts of a photovoltaic system other than the solar modules, such as mounting, cables, connectors, protection devices, charge controllers, inverters, batteries and monitoring equipment.

All system components and costs other than the PV modules.

An electrochemical energy-storage device made from one or more cells. In solar systems, batteries store energy for use when production is insufficient or unavailable.

Two or more batteries connected together to provide the required system voltage, capacity or both.

The amount of charge or energy a battery can store. It is commonly stated in ampere-hours (Ah) and may also be expressed in watt-hours (Wh) or kilowatt-hours (kWh).

See “Cell”.

The number of charge-and-discharge cycles a battery can complete before its usable capacity falls to a specified level. Cycle life depends strongly on depth of discharge, temperature, charging conditions and battery chemistry.

An electronic protection and control system used with lithium batteries. A BMS monitors cells and may limit or disconnect charging and discharging when voltage, current or temperature moves outside safe limits.

The gradual loss of stored charge while a battery is not supplying a load. The rate depends on chemistry, temperature, age and battery condition.

Percentage of full charge, or 100 percent minus the depth of discharge.

Battery current.

A diode used to prevent unwanted reverse current, for example current flowing from a battery back toward a solar array when production is low.

Balance of system.

Business Opportunities with Solar Systems: solar systems that can be used for income generation in non-electrified areas.

Base Transceiver Station: the antenna and radio equipment necessary to provide mobile service in an area.

The main charging stage in which a charger supplies as much permitted current as possible until the battery reaches the next charging-voltage threshold.

A conductive bar or block used to distribute high current between batteries, inverters, chargers, loads and protection devices. It must be correctly rated and protected.

A diode connected across a group of solar cells so current can bypass that group when it is shaded or damaged. It helps reduce hot-spot risk and power loss but does not eliminate the effect of shading.

A way to express charge or discharge current relative to battery capacity. For a 100 Ah battery, 1C is 100 A, 0.5C is 50 A and 0.1C is 10 A.

The reduction in voltage caused by current flowing through a cable's resistance. Voltage drop increases with current and cable length and decreases when a larger conductor cross-section is used.

Cathodic protection (CP) is a technique used to control corrosion of a metal surface by making it the cathode of an electrochemical cell. It is used for structures such as steel water or fuel pipelines, storage tanks, pier piles, ships, boats, offshore platforms, well casings and metal reinforcement in concrete.

The smallest electrochemical unit within a battery or the basic light-sensitive element within a solar module. Multiple cells are connected to obtain the required voltage and current.

A device placed between a solar array and battery to regulate charging. It must be compatible with the battery voltage and chemistry and must remain within its PV voltage, current and power limits. See the Solar Charge Controllers Guide.

The current applied to a cell or battery to restore its available capacity.

The sequence of charging voltages, current limits and stages required by a battery chemistry. Using the wrong profile can reduce performance, shorten service life or create a safety risk.

A thin-film solar cell based on copper, indium and selenium (CIS), or copper, indium, gallium and selenium (CIGS).

Club of Renewable Electrification Agencies.

A PV module that uses optical elements to increase the amount of sunlight incident on a PV cell. Concentrating arrays must track the sun and use direct sunlight because diffuse light cannot be focused onto the PV cells.

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

The flow of electric charge, measured in amperes (A).

The number of complete or partial charge-and-discharge cycles a battery can deliver before reaching a defined end-of-life capacity.

The number of days a battery bank is intended to supply the planned loads without meaningful energy input from solar or another charging source.

An electronic circuit that converts DC voltages, for example PV-module voltage, into other levels such as load voltage. It can form part of a maximum power point tracker (MPPT).

An electronic device that converts one DC voltage level to another, for example from a 24 V battery system to a regulated 12 V load supply.

A type of battery that can be discharged to a large fraction of its capacity many times without damaging the battery.

A battery designed to be repeatedly discharged and recharged. It is more suitable for regular off-grid loads than a starter battery designed mainly for short high-current engine starts.

The percentage of a battery's nominal capacity that has been removed. A battery at 80% state of charge has approximately 20% depth of discharge.

Depth of discharge is a measure of how deeply a battery is discharged. When a battery is 100% full, DOD is 0%. When it is 100% empty, DOD is 100%. Deeper average discharge generally reduces cycle life.

Electric current that flows in one direction. Solar modules and batteries produce DC electricity.

The current or power taken from a battery over time. High discharge rates can reduce usable capacity, increase heating and affect voltage.

A mounting system that moves a solar array around two axes to follow both the direction and elevation of the sun.

A system capable of rotating independently around two axes and following the sun's direction and height in the sky.

The proportion of time that a device operates within a given period. A refrigerator compressor, pump or fan may operate intermittently rather than continuously.

The ratio of useful output energy or power to input energy or power, normally expressed as a percentage.

The flow of electric charge through a conductor or another medium.

The ion-conducting material inside a battery cell. In flooded lead-acid batteries it is a liquid sulfuric-acid solution; in AGM and Gel batteries it is immobilised.

The ability to perform work. Electrical energy is commonly measured in watt-hours (Wh) or kilowatt-hours (kWh).

A coordinated system that stores electrical energy and manages charging, discharging and power flow. It may include batteries, inverter/chargers, monitoring, protection and control equipment.

A controlled high-voltage charging procedure used only with battery types and settings that explicitly allow it, typically some flooded lead-acid batteries. It must not be applied to sealed or lithium batteries unless the manufacturer specifically requires it.

A controlled overcharge procedure intended to address sulphation in compatible lead-acid batteries. The process can cause the electrolyte to gas and must be performed only with the correct battery type, supervision and precautions.

Fluorescent compact lamp.

A maintenance charging stage that holds a fully charged lead-acid battery at a lower voltage to counter self-discharge without continuous heavy charging.

A sacrificial overcurrent-protection device that opens a circuit when current exceeds its rating for long enough. DC fuses must have a suitable DC voltage and interrupt rating.

The portion of charging current that produces hydrogen and oxygen from the battery electrolyte. This current increases as voltage and temperature increase.

A sealed VRLA lead-acid battery in which the electrolyte is held in a silica gel. Gel batteries can suit deep-cycle applications but require compatible charging voltages and current limits.

A lead-acid battery in which the electrolyte is held in a silica-gel matrix.

A machine that converts mechanical energy into electrical energy. In an off-grid system, an engine generator may provide backup power or battery charging when renewable production is insufficient.

One billion watts, one million kilowatts or one thousand megawatts.

The sum of the direct and diffuse components of solar radiation.

The total solar energy received on a surface from direct and diffuse sunlight over a stated period, commonly expressed in kWh/m².

Global System for Mobile Communications.

A multifunction device that can combine battery inversion, AC battery charging, transfer switching and solar MPPT charging. Available functions and supported operating modes vary by model.

A power system that combines more than one energy source, such as solar, wind, a generator or the utility grid, often with battery storage.

A graph showing the relationship between current and voltage from a photovoltaic device under stated conditions. The maximum power point lies where voltage × current is highest.

A two-digit enclosure rating. The first digit indicates protection against solid objects and dust; the second indicates protection against water. The complete rating and installation instructions must be checked.

Solar radiation received by a surface. The term is often used for solar energy over time and should not be confused with insulation.

A conductor within a module, or another means of connection, that provides an electrical interconnection between solar cells.

A device that converts DC electricity from a battery or other DC source into AC electricity for AC loads. See the Solar Inverter Guide.

A device that combines an inverter with an AC-powered battery charger and usually an automatic transfer switch.

Devices that convert DC electricity into AC electricity, either for stand-alone systems or for utility-interactive systems.

Ingress Protection.

The joule is the SI unit of energy and work. One joule is the work done when a force of one newton moves an object one metre in the direction of the force.

The SI unit of energy. One watt-second equals one joule.

An enclosure used to contain and protect electrical connections. A solar module junction box normally contains the module output connections and bypass diodes.

A unit of power equal to 1,000 watts.

One thousand watt-hours.

A measure of the nominal peak power of a photovoltaic device under laboratory illumination conditions, normally standard test conditions (STC).

A unit of energy equal to 1,000 watt-hours. One kilowatt used continuously for one hour consumes one kilowatt-hour.

The combined rated peak power of a photovoltaic array under standard test conditions.

A rechargeable battery chemistry using lead-based plates and a sulfuric-acid electrolyte. Common types include flooded, AGM and Gel batteries.

An electrical storage device that uses a reversible chemical reaction involving lead plates or grids and a diluted sulphuric-acid electrolyte to convert electrical energy into chemical energy and back again.

Lithium-ion.

Lithium Cobalt Oxide.

An estimate of the cost of owning and operating a system over its useful life, usually expressed as the present value of all lifetime costs.

Lithium iron phosphate.

Lithium iron phosphate, a lithium-ion battery chemistry widely used for off-grid storage because of its high usable capacity, low weight and long cycle life when correctly managed.

A family of rechargeable battery chemistries that move lithium ions between electrodes. Solar batteries require a compatible BMS, charger settings and system design.

Lumen.

Any device or circuit that consumes electrical power.

A function or device that disconnects loads when battery voltage falls below a set threshold, helping prevent excessive discharge. Voltage alone is not always an accurate measure of remaining battery capacity.

The operating point on a solar module's I-V curve where voltage multiplied by current produces the greatest power.

A power-conditioning device that automatically operates the PV generator at its maximum power point under changing conditions.

A control method that continually adjusts the PV operating point to extract available power and converts it to the voltage and current required by the battery or DC bus.

One million watts, or 1,000 kilowatts.

A weather-protected assembly of interconnected photovoltaic cells, commonly called a solar panel.

Photovoltaic cells made from single-crystal silicon. Modern monocrystalline modules are widely used because they provide good power density and are available in many formats.

Mobile Telephone Networks.

See “Poly-crystalline”.

An abbreviation for the National Electrical Code®, which contains safety guidelines and required practices for electrical installations.

Nickel-cadmium.

Nickel-metal hydride.

A reference operating temperature measured under specified conditions that helps estimate module behaviour above laboratory cell temperature. Some datasheets use the older term NOCT; always follow the conditions stated by the manufacturer.

A reference solar-cell or module operating temperature stated in manufacturer literature under specified operating conditions.

A reference value used to describe a battery, module or system class, such as 12 V, 24 V or 48 V. Actual operating and charging voltages differ from the nominal value.

A power system that operates independently of the public utility grid. It may use solar, wind, generators, batteries or a combination of sources.

An autonomous photovoltaic or hybrid system that is not connected to the utility grid. Most off-grid systems use batteries or another form of energy storage.

The unit of resistance to the flow of electric current.

The unit of electrical resistance.

The voltage measured across a solar module or array when no current is flowing. Array Voc rises in cold conditions and must remain below the connected controller or inverter's maximum PV voltage.

A tubular-plate lead-acid battery with liquid electrolyte.

A tubular-plate lead-acid battery with gel electrolyte.

The compass direction a solar module or array faces.

Partial loss.

A connection in which positive conductors are joined together and negative conductors are joined together. For matched solar modules or batteries, voltage remains approximately the same while current or capacity increases.

The highest power demand expected at one time. Inverter sizing must consider both continuous load and short starting surges.

The maximum electrical load or power demand occurring within a given period, typically a day.

The equivalent number of hours per day at a solar irradiance of 1,000 W/m² that would provide the same daily solar energy as the actual varying conditions.

The direct conversion of light into electricity using semiconductor materials.

One or more interconnected photovoltaic modules arranged to provide the required power, voltage and current.

A weather-protected assembly of solar cells and electrical connections designed to generate DC electricity from sunlight. See the Solar Panels Guide.

The maximum rated output of a photovoltaic cell, module or system under defined rating conditions.

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance-of-system components.

A small solar home system with a power output of approximately 1–10 W.

A material composed of many small, differently oriented crystals. The grain boundaries between crystallites can reduce photovoltaic conversion efficiency.

The rate at which energy is generated, transferred or used. Electrical power is measured in watts and is calculated as voltage × current in a DC circuit.

The ratio of real power in watts to apparent power in volt-amperes in an AC circuit. Loads with a low power factor can require more inverter current than their watt rating alone suggests.

A charge controller that regulates battery charging by rapidly switching the PV connection. It is most suitable when the solar module's operating voltage is closely matched to the battery charging voltage.

An inverter that uses pulse-width modulation to produce an output waveform close to a sine wave with reduced current harmonics.

An AC waveform closely matching utility-grid power. Pure-sine-wave inverters are normally preferred for sensitive electronics, motors and appliances.

Abbreviation for photovoltaic.

IEA Photovoltaic Power Systems Programme, an international technology collaboration programme focused on photovoltaic power systems.

Pulse-width modulation.

The manufacturer's stated power under defined test or operating conditions. Real output can differ because of temperature, irradiation, voltage, efficiency and other conditions.

Opposition to electric current, measured in ohms. Resistance in cables and connections causes voltage drop and heat.

The property of a conductor that opposes electric-current flow and causes heat to be generated. Resistance is measured in ohms.

Revolutions per minute.

A material with a limited and controllable capacity for conducting electric current. Photovoltaic semiconductors include silicon, gallium arsenide, copper indium diselenide and cadmium telluride.

A connection in which the positive terminal of one device is connected to the negative terminal of the next. For matched solar modules or batteries, voltage adds while current or ampere-hour capacity remains approximately the same.

A battery charge regulator in which charging current is controlled by a switch connected in series with the PV module or array.

The current produced by a solar module or array when its output terminals are shorted under stated test conditions. Isc is used when checking controller, cable and protection ratings.

Solar home system.

A precision low-resistance device installed in a DC circuit so a battery monitor can measure current flow and calculate energy entering and leaving the battery.

A battery charge regulator in which charging current is controlled by a switch connected in parallel with the PV generator. Battery overcharging is prevented by shunting the PV generator.

A chemical element with the symbol Si and atomic number 14. Very pure silicon is the basis of most photovoltaic cells.

An inverter that produces a utility-quality sine-wave output.

Sealed Lead Acid.

Solar module.

The angle between the horizon and the sun's position in the sky.

The complete photovoltaic generating part of a system, including one or more solar modules and their electrical arrangement.

A simple inverter topology that produces a square-wave AC output. It is inexpensive but supplies lower-quality power than a sine-wave inverter.

See “Off-Grid”.

The laboratory reference conditions used for solar-module nameplate ratings: 1,000 W/m² irradiance, 25°C cell temperature and an air mass of 1.5.

An estimate of how much usable charge remains in a battery, normally expressed as a percentage of full charge.

A measure of how full a battery is. The relationship between battery voltage and state of charge depends on battery chemistry, temperature, load and recent charging conditions.

A group of solar modules connected in series. String voltage is the sum of the module voltages, while string current is approximately the current of one matched module.

Formation of lead-sulfate crystals on lead-acid battery plates. Prolonged partial charge or deep discharge can make the crystals harder to reverse and reduce battery capacity.

The momentary start-up condition of a motor that requires a large amount of electrical current.

The ability of an inverter or generator to supply the high current briefly required when a motor starts.

The short-duration power an inverter or generator can supply above its continuous rating, often required when motors, compressors or pumps start.

A person or company specialising in bringing component subsystems together into a complete system and ensuring that the subsystems function correctly together.

The nominal DC voltage around which a battery-based system is designed, commonly 12 V, 24 V or 48 V.

The percentage change in a solar module's voltage, current or power for each degree of temperature change from the reference condition. Module voltage normally increases as cell temperature falls.

Automatic adjustment of lead-acid charging voltage according to battery temperature. Lithium charging requires chemistry-specific temperature rules instead.

A very thin layer of semiconductor material, such as copper indium diselenide, cadmium telluride, gallium arsenide or amorphous silicon, used to manufacture photovoltaic cells.

A photovoltaic technology in which very thin semiconductor layers are deposited onto a substrate. It is used in some lightweight, flexible and specialised modules.

The angle between a solar module and the horizontal surface.

A mounting method that moves solar modules to follow the sun. Tracking can increase energy yield but adds cost, moving parts and installation requirements.

An electromagnetic device that transfers AC energy between circuits and can change voltage or provide electrical isolation.

A system that supplies power to connected loads when the normal AC source fails. Battery-backed inverter/chargers can provide UPS or backup functions, subject to transfer time and product design.

The portion of nominal battery energy that can be withdrawn within the manufacturer's operating limits. It depends on chemistry, depth of discharge, current, temperature and battery condition.

Universal Serial Bus.

An inverter designed to operate in coordination with the public grid. Grid-connected operation must comply with applicable local connection rules and product certifications.

Voltage measured in an alternating-current circuit.

Voltage measured in a direct-current circuit.

Volt-ampere.

Volts AC.

A sealed lead-acid battery design with pressure-regulating valves and internal gas recombination. AGM and Gel are the two common VRLA types.

Volts DC.

The voltage of a solar module or array at its maximum power point under stated conditions.

The open-circuit voltage of a solar module or array when no current is flowing. Cold-weather Voc must be checked before connecting modules in series.

The unit of electrical potential difference.

A unit of apparent AC power. Inverter ratings may be stated in VA, while the usable real power in watts depends on the equipment and load power factor.

Electrical potential difference between two points. Voltage is one of the main specifications used to determine component compatibility.

The reduction in voltage along cables, connectors, switches or protection devices because of their resistance.

Valve-Regulated Lead-Acid. AGM and Gel are the two common VRLA battery types.

The unit of power. In a DC circuit, watts are calculated as volts × amperes.

A unit of energy equal to one watt used or produced for one hour.

The shape of a varying electrical signal over time. Common inverter output descriptions include pure sine wave, modified sine wave and square wave.

The amount of electrical energy produced by a solar system over a stated period, commonly expressed in Wh, kWh or kWh per installed kWp.