Let’s start with the basics: what is electricity, and where does it come from?

Electricity is energy used to perform work, like running your appliances or charging an electric vehicle. Solar energy harnesses photons, which are energy in the form of light, and uses photovoltaic panels ("photo" meaning light and "voltaic" referring to electricity) to convert them into electricity with the help of semiconductors..

Historically, electricity has been generated by turning turbines. For instance, coal, natural gas, and other fossil fuels are burned to create heat, which creates steam that then moves the blades of a turbine to create electricity. With hydroelectric and wind power, flowing water or moving air is used to turn turbine blades, and the energy from the blades is converted into electricity using a generator. Nuclear power uses fission to create heat that eventually becomes electricity.

Unfortunately, burning coal, oil, and natural gas emits large amounts of carbon dioxide (CO2), worsening climate change. Mining uranium for nuclear energy has its own challenges, and unlike solar energy, these sources are also finite.

How electricity is delivered

Once generated, electricity enters a vast network of high-voltage power lines called the transmission grid—also called the utility grid or, simply, the grid. These lines move electricity from generation points to the places where energy is used.

The high-voltage power moves through a series of substations and transformers, which step down the power to voltages that can be used by homes and businesses. The energy is distributed through wires to your neighborhood and then into your home’s main electrical service panel, which distributes it to various circuits in your home to power your lights, appliances, and more.

Main service panels

Your home’s main service panel determines how much electrical capacity your home can handle, generally 100 or 200 amps. Depending on the age and capacity—and future expected electrical demand—your main panel may need to be upgraded to handle the additional electrical current from solar and battery storage. But with Enphase Power Control technology, many homeowners avoid that expense. This innovative feature can efficiently manage your electrical loads and help maximize your solar production.

You’re likely already familiar with your breaker panel. It’s usually rectangular in shape, with a metal door and a number of switches, or breakers, inside. These circuits are typically rated at 15 or 20 amps, with higher-demand appliances like air conditioners or EV chargers connected to 30 to 50 amp circuits. (See more about circuits and amps in the ‘AC vs DC power’ section below.)

When a circuit exceeds its capacity, the breaker trips to prevent overheating or potential fire hazards. Your main service panel door likely features labels, added by an electrician, indicating which outlets and appliances are connected to each circuit. By understanding the distribution of loads across your circuits, you can prevent overloading by redistributing appliances to balance the load more effectively.

Electric meters

Your utility uses your electric meter, usually located outside your home, to track your home's energy use.

There are two basic types of electric meters: smart meters and analog meters. Analog meters measure usage with dials but cannot measure energy exports from solar. Many utilities are upgrading to smart meters, which track both imports and exports digitally and allow the utility to remotely track energy flow. Smart meters also allow you to view detailed energy usage data online. If you're on a time-of-use rate plan, this data can help you shift consumption to cheaper rate periods, so you save money. You just need to set up an online account with your utility. (This information will be useful when you go solar since your installer will use it to help size your system.)

Kilowatts vs. kilowatt-hours  (or power vs. energy)

There are two units of measurement of your home’s electricity usage that are important to understand: 

Kilowatts (kW): Measures how much power you use at a specific moment. A kilowatt is 1,000 watts.

Kilowatt-hours (kWh): Measuring your energy usage over time, kWh is the measure your utility uses to bill you for your electricity. A kilowatt-hour is 1,000 watt-hours.

For example, running your air conditioning, lights, and microwave might consume 5 kilowatts of power at a given moment. But once your microwave turns off, over the course of a full hour, you may only use 4 kilowatts total. So your utility would bill you for 4 kilowatt-hours.

Think of it like driving. You may be doing 60 at a given moment, but over the course of an hour, your average speed may actually be 56. Translating that to electricity usage, 60 is the measure of kilowatts you’re using at a given moment, while 56 is how many kilowatts you used for a full hour, or 56 kilowatt-hours.

AC vs. DC power

AC (alternating current) is the type of electricity your home uses. Its ability to change direction allows it to travel efficiently over power lines and power household appliances.

DC (direct current) flows in one direction and is stored in batteries like those in phones, EVs, or solar energy systems.

Most of your home's outlets will be three-prong, grounded outlets delivering AC power at 120 volts for standard appliances and electronics. They will all be wired into your main electrical panel.

High-demand appliances and electronics like clothes dryers, electric stoves, air conditioning, and Level 2 EV chargers require more power than a simple 120 V outlet can deliver. So you’ll likely have 240 V or 250 V outlets in your garage or behind your electric stove. They’ll be wired with heavier-gauge electrical wire and connected to higher-voltage circuits—usually 40 to 50 A, or more for faster-charging EV chargers like the Enphase IQ 60 EV Charger or IQ 80 EV Charger, which require 80 A and 100 A circuits, respectively.

We didn’t include the sun as a source of electricity in the section above because it works differently than traditional power sources like coal and natural gas. Instead of driving turbines, it directly captures sunlight to generate electricity.

How solar panels work

The sun's abundant, renewable, and unlimited energy provides the potential to power your home every single day—even if it’s cloudy. Solar panels, mounted on your home or on a ground mound, use photovoltaic (or PV) cells to absorb sunlight and convert it to direct current (DC) electricity.

However, DC electricity isn't what your home uses, so it needs to be converted to alternating current (AC) electricity. That's where inverters and microinverters, like our Enphase IQ Microinverters, come in. They convert DC from the panels into AC, which then flows to your main service panel and is distributed throughout your home to power your fridge, TV, Wi-Fi, and anything else that requires electricity to run.

The depends, because there are quite a few factors that affect power production. These include the system size (the number of panels); the efficiency of the panels (which can vary by manufacturer and model); the orientation of your roof to the sun; the amount of shading from trees or clouds; and even your location, with production changing from season to season as the amount of sunlight changes.

Can you replace your electric utility with solar?

You can, but it's not always the best move.

To replace your electric utility with solar and go completely off-grid, you’d need to have enough solar production year-round to power all your electric appliances, including charging an EV if you have one or plan to get one. And you’d need enough battery storage to power all of your electricity needs overnight. But a true off-grid system, with a larger solar array and more battery storage, is too expensive—and not necessary—for most people.

Staying connected to your utility grid has important benefits that you'd miss out on if you go off-grid: it allows you to draw supplemental power from the grid if you need it, and in most places, it allows you to sell excess solar power back to the utility to earn you bill credits.

Ideally, your solar system will produce enough energy to cover what you use over the course of a year. What that means will vary from state to state, and in some cases, utility to utility. In states with favorable net metering structures (where you're paid for excess energy production at a rate equal or close to what you pay them), then bill credits from your excess production should cover the cost of energy you draw from the grid over the course of the year.

However, in states like California, where, under NEM 3.0, you get paid a fraction of the retail rate for excess production, you may want to invest in batteries to store your excess solar production to use once the sun sets. Ideally, your solar produces enough energy to cover your usage during the day plus charge your batteries, then your batteries power your home at night so you don't have to import expensive electricity from the grid. You essentially get free electricity for the year, minus any typical connection or other fees your utility charges.

Even if your system doesn’t cover 100% of your energy usage, every kilowatt-hour it produces is still saving you money on energy costs.

Your solar energy system can range from basic to comprehensive. For starters, your system will include solar panels and either string inverters or microinverters, which will connect to your main electrical panel and your utility meter.

Types of solar panels

There are three main types of solar panels, and your installer will help you decide which panels are best for your roof and your needs:

Monocrystalline panels: Highly efficient (18-22%), they convert a higher percentage of sunlight into electricity and usually come with 25-year warranties. They cost a bit more than polycrystalline panels, but because of their efficiency, you don't need as many panels to produce the same amount of energy.

Polycrystalline panels: Slightly less efficient (15-18%) but more affordable than monocrystalline panels, these also usually come with 25-year warranties. The drawback is that they require more roof space to produce the same amount of energy as monocrystalline panels.

Thin-film panels: Lightweight and flexible, these have lower efficiency (10-12%) but are suitable for large, flat surfaces or areas where weight is a concern. However, they do degrade faster and may need replacement in 10 to 15 years.

Panel racking systems

Your solar panels will also need a racking system to attach to your roof (or, if you choose, for ground-mounting).

There are different mounting systems for different types of roofs. Installers will usually secure mounting hardware to the rafters of your roof, using flashing to prevent any leakage from rain or snowmelt. There are also some racking systems for terra cotta roofs that don’t require drilling. Regardless of the mounting solution, making sure your roof is still fully watertight should be part of the installer's warranty and guarantee.

Once the mounts are in place, rails are attached and the panels are secured to the rails with clamps or other hardware. Considerations are made for roof load capacity to ensure it can handle the panel and racking system and any potential snow load, as well as ensuring your system stays in place during seismic activity, high winds, and even hurricanes.

Local municipal or HOA codes may further dictate the type of racking system you need, as well as setbacks and any other structural requirements.

Batteries

Adding battery storage, such as Enphase IQ Batteries, to your system allows you to use stored solar energy after sundown and avoid buying expensive electricity from the grid. These batteries can also be configured to provide backup power to help protect you from grid outages. (You can have backup batteries without solar, as well.)

EV chargers

If you have a plug-in vehicle, you can include an EV charger in your system. With a smart solar charger like the Enphase IQ EV Charger, you'll be able to charge your car in part or completely with solar energy. And with systems that include energy management solutions like the Enphase App, you can easily integrate, monitor, control, and customize everything with your phone.

Home electrification

To take even greater advantage of the clean, free electricity from your solar energy system, you can replace gas appliances like stoves, water heaters, and dryers with electric versions. You can also convert from a natural gas furnace to an efficient electric heat pump that also cools your home on the hot days of summer.

Generators

You can also add a backup generator, which can improve resilience in case your solar and battery system isn’t adequate due to extended poor weather or unusually high power use. However, they can be noisy, and require a fuel source like gas or diesel, safe fuel storage, and regular operation and maintenance to stay in good working order.

Solar energy is a long-term investment. High-quality solar energy systems are designed for durability and longevity, with most solar PV panels warrantied for 25 years. But they can continue producing long after that—some for up to 40 years.

Lifespans of inverters can vary depending on the type of inverter. Traditional string inverters often come with warranties of 10 to 12 years. Enphase IQ Microinverters, which mount directly on the panels, are designed to last as long as the panel themselves. They’re backed by a 25-year warranty, but designed to keep producing for years beyond the warranty. This is critical since inverters are necessary for the panels to produce usable energy.

Most batteries are also designed to work longer than their warranties, which usually run around 10 years, though we back our newest Enphase IQ Batteries for an industry-leading 15 years.

Can solar panels and batteries be recycled?

This is a great question with an even better answer: yes! In fact, California and Washington have both passed laws mandating panel recycling, while several other states are exploring similar rules.

The challenge has been that there weren’t many companies recycling either panels or batteries, but that’s recently changed. There are new companies that can disassemble panels and recover their components for reuse or recycling. The same goes for batteries—which is especially important because the highly valuable lithium, nickel, cobalt, and manganese in older batteries can be extracted and reused in new batteries. Plus, the plastic, aluminum, and copper from the battery body and wiring can also be recycled. These efforts are helping make future production more sustainable.

We understand that the safety of your solar system, home, and family is a concern when installing advanced electronics on your roof and batteries in your garage.

String inverter-based solar PV systems carry risks such as arcing from high-voltage DC power at the panels, which could lead to fires. Developing safer solar technology was one of the prime drivers for the invention of the Enphase microinverter.

In an Enphase Energy System, an Enphase microinverter is installed on the back of every panel to convert the DC generated by the panel to AC like you use in your home. This unique design virtually eliminates the risk of high-voltage DC arc fault fires, and it’s very uncommon for AC to arc and create a safety hazard. Plus, built-in Enphase rapid shutdown technology ensures the entire system can be quickly de-energized by turning off a single breaker.

That allows you or first responders to quickly shut off the system, minimizing fire danger and helping them more safely address any emergency. Systems using string inverters require numerous additional components to comply with rapid shutdown requirements.

Enphase subjects IQ Microinverters and IQ Batteries to extreme testing to identify and eliminate any issues before they enter the field. Once installed, Enphase monitors equipment remotely 24/7 to ensure everything is operating as expected.

Plus, Enphase IQ Batteries use lithium iron phosphate (LFP) technology that's safer than more volatile battery chemistries like nickel manganese cobalt (NMC). They've also been tested and refined so extensively that they became the first microinverter-based batteries to meet the performance criteria of UL 9540A—a unit-level test for protection against the spread of a thermal runaway fire in residential indoor wall-mounted systems.

All that means an Enphase Energy System isn’t just sustainable—you're assured that it will be safe and reliable for years to come, too.

Learn more about Enphase safety.

Alternating current (AC): A type of electrical current, the direction of which is reversed at regular intervals or cycles. Electricity transmission networks use AC because voltage can be controlled with relative ease.

Ambient temperature: The temperature outside in your area. (Ambient temperature affects solar production.)

Amorphous silicon: A thin‐film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single‐crystal silicon and polycrystalline silicon.  

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.

Angle of incidence: The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface – like a solar panel – that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°. The angle at which the sun strikes your panels will affect panel output.

Availability: The quality or condition of a photovoltaic system available to provide power to a load; usually measured in hours per year.  

Backup: A power source—usually either a solar battery or generator—designed to provide power to your home (or business) in the event of a power outage.

Battery: 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.  

Battery available capacity: The total maximum charge, expressed in ampere‐hours, that can be withdrawn from a cell or battery under a specific set of operating conditions, including discharge rate, temperature, 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 everything.  

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), which can be withdrawn from a fully charged cell or battery. That capacity can vary with temperature, rate, age and cut‐off voltage.  

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 intended service type.

Cell (battery): A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected to produce higher voltages.  

Charge: The process of adding electrical energy to a battery.

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

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

Current at maximum power (Imp): The current at which maximum power is available from a module.  

Cycle: The discharge and subsequent recharge of a battery.

Deep‐cycle battery: A battery with large plates that can withstand many discharges to a low state‐of‐charge.  

Deep discharge: Discharging a battery to 20% or less of its full charge capacity.

Depth of discharge (DoD): The ampere‐hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, removing 25 ampere‐hours from a fully charged 100 ampere‐hours‐rated cell results in a 25% depth of discharge.  

Direct current (DC): A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120-volt or 220‐volt household appliances, DC must be converted to alternating current (AC), its opposite.

Discharge rate: The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.  

Disconnect: Switch gear used to connect or disconnect components in a photovoltaic system.  

Distributed energy resources (DER): A variety of small, modular power‐generating technologies that can be combined with energy management and storage systems like batteries, and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid.  

Distributed power: Generic term for any power supply located near the point where the power is used. Opposite of central power.  

Electric circuit: The path followed by electrons from a power source (generator or battery), through an electrical system and back to the source.  

Electric current: The flow of electrical energy (electricity) in a conductor, measured in amperes.  

Electrical grid: An integrated system of electricity distribution, usually covering a large area.  

Electricity: Energy resulting from the flow of charged particles, such as electrons or ions.

Electrode: A conductor that is brought in conducting contact with a ground.

Electrolyte: A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.  

Electron: An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom.  

Energy: The capability of doing work; different forms of energy can be converted into other forms but the total amount of energy remains the same.  

Energy audit: A survey that shows how much energy used in a home and helps identify ways to use less energy.

Equinox: The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23rd (fall equinox).

Fill factor: A key characteristic in evaluating cell performance, it’s the ratio of a photovoltaic cell's actual power to its power if both current and voltage are at their maximum.  

Fixed tilt array: A photovoltaic array set in at a fixed angle with respect to horizontal.

Frequency: The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).  

Frequency regulation: This indicates the variability in the output frequency. Some loads will switch off or not operate properly if frequency variations exceed one percent.

Full sun: The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).

Gel‐type battery: Lead‐acid battery in which the electrolyte is composed of a silica gel matrix.  

Gigawatt (GW): A unit of power equal to 1 billion watts; 1 million kilowatts or 1,000 MWs.

Grid‐connected system: A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying excess power back to the grid.

High voltage disconnect: The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging

Hybrid system: A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

Incident light: Light that shines onto the face of a solar cell or module.

Input voltage: This is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage.  

Insolation: The solar power density incident on a surface of stated area and orientation, usually expressed as watts per square meter or BTU per square foot per hour.  

Interconnection (to grid): The process of finalizing the connection of a solar array to the electrical grid.

Inverter: A device that converts DC electricity to AC either for stand‐alone systems or to supply power to an electricity grid.  

Ion: An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.  

Irradiance: The direct, diffuse and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter.  

Joule: A metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 BTU equals 1,055 joules.

Junction box: A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

Kilowatt (kW): A standard unit of electrical power equal to 1,000 watts, or to the energy consumption at a rate of 1,000 joules per second.  

Kilowatt‐hour (kWh): 1,000 watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.

Lead‐acid battery: A general category that includes batteries with plates made of pure lead, lead‐antimony, or lead‐calcium immersed in an acid electrolyte.  

Life: The period during which a system is capable of operating above a specified performance level.  

Life‐cycle cost: The estimated cost of owning and operating a photovoltaic system for the period of its useful life.

Line‐commutated inverter: An inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there’s a failure in the power grid, the photovoltaic system cannot feed power into the line.  

Liquid electrolyte battery: A battery containing a liquid solution of acid and water.  

Load: The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.  

Load circuit: The wire, switches, fuses, etc. that connect the load to the power source.  

Load current (A): The current required by the electrical device.                    

Load resistance: The resistance presented by the load.  

Low voltage cutoff (LVC): The voltage level at which a charge controller will disconnect the load from the battery.  

Low voltage disconnect: The voltage at which a charge controller will disconnect the load from the batteries to prevent over‐discharging.  

Low voltage disconnect hysteresis: The voltage difference between the low voltage disconnect set point and the voltage at which the load will be reconnected.  

Low voltage warning: A warning buzzer or light that indicates the low battery voltage set point has been reached.  

Maintenance‐free battery: A sealed battery to which water cannot be added to maintain electrolyte level.

Megawatt (MW): 1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.  

Megawatt‐hour (MWh): 1,000 kilowatt‐hours or 1 million watt‐hours.

Modularity: The use of multiple inverters connected in parallel to service different loads.

Multi-crystalline: A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.  

Multijunction device: A high‐efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.  

Multi‐stage controller: A charging controller unit that allows different charging currents as the battery nears full state‐of‐charge.  

Ohm: A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.  

One‐axis tracking: A system capable of rotating about one axis.  

Operating point: The current and voltage that a photovoltaic module or array produces when connected to a load.  

Operating point: The current and voltage that a photovoltaic module or array produces when connected to a load.  

Parallel connection: A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.  

Peak demand/load: The maximum energy demand or load in a specified time period.

Peak power current: Amperes produced by a photovoltaic module or array operating at the voltage of the I‐V curve that will produce maximum power from the module.  

Peak power point: Operating point of the I‐V (current‐voltage) curve for a solar cell or photovoltaic module where the product of the current value times the voltage value is a maximum.  

Peak power tracking: See maximum power tracking.  

Peak sun hours: The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2.

Peak watt: A unit used to rate the performance of solar cells, modules or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.  

Phosphorous (P): A chemical element used as a dopant in making n‐type semiconductor layers.  

Photocurrent: An electric current induced by radiant energy.

Photon: A particle of light that acts as an individual unit of energy.  

Photovoltaic(s) (PV): Pertaining to the direct conversion of light into electricity.  

Photovoltaic (PV) array: An interconnected system of PV modules that function as a single electricity‐producing unit.  

Photovoltaic (PV) cell: The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.  

Photovoltaic (PV) conversion efficiency: The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.  

Photovoltaic (PV) device: A solid‐state electrical device that converts light directly into direct current electricity of voltage‐current characteristics that are a function of the characteristics of the light source and the materials in and design of the device.  

Photovoltaic (PV) effect: The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

Photovoltaic (PV) generator: The total of all PV strings of a PV power supply system, which are electrically interconnected.  

Photovoltaic (PV) module: The smallest environmentally protected, essentially planar, assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight.  

Photovoltaic (PV) panel: Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules.  

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

Photovoltaic‐thermal (PV/T) system: A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form.  

Plates: A metal plate, usually lead or lead compound, immersed in the electrolyte in a battery.  

Pocket plate: A plate for a battery in which active materials are held in a perforated metal pocket.  

Point‐contact cell: A high-efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point‐diffused contacts on the rear surface for current collection.

Polycrystalline silicon: A material used to make photovoltaic cells, which consist of many crystals, unlike single‐crystal silicon.  

Power conditioning: The process of modifying the characteristics of electrical power (e.g., for inverting DC to AC).  

Power conditioning equipment: Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.

Power conversion efficiency: The ratio of output power to input power of the inverter.  

Power factor (PF): The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that’s apparently being drawn from a power source, expressed in volt‐amperes or kilovolt‐amperes.  

Primary battery: A battery whose initial capacity cannot be restored by charging. 

Projected Area: The net south‐facing glazing area projected on a vertical plane.

Qualification test: A procedure applied to a selected set of photovoltaic modules involving the application of defined electrical, mechanical or thermal stress in a prescribed manner and amount.  

Rated battery capacity: The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature.  

Reverse current protection: Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night).  

Ribbon (photovoltaic) cells: A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

Schottky barrier: A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

Sealed battery: A battery with a captive electrolyte and a resealing vent cap, also called a valve‐regulated battery. Electrolyte cannot be added.

Series connection: A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.  

Series controller: A charge controller that interrupts the charging current by open‐circuiting the photovoltaic (PV) array.  

Series regulator: Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.  

Series resistance: Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.  

Shallow‐cycle battery: A battery with small plates that cannot withstand many discharges to a low state‐of‐charge.  

Silicon (Si): A semi‐metallic chemical element that makes an excellent semiconductor material for photovoltaic devices.  

Solar constant: The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1,353 Watts per square meter or 492 BTU per square foot.  

Solar cooling: The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can also power evaporative coolers (swamp coolers), heat pumps and air conditioners.  

Solar energy: Electromagnetic energy transmitted from the sun (solar radiation).  

Solar‐grade silicon: Intermediate‐grade silicon used in the manufacture of solar cells. Less expensive than electronic‐grade silicon.  

Solar noon: The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky; equal to true or due geographic south.  

Solar resource: The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.  

Solar spectrum: The total distribution of electromagnetic radiation emanating from the sun.  

Solar thermal electric systems: Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator.  

Stand‐alone system: An autonomous or hybrid photovoltaic system not connected to a grid. Most stand‐alone systems require batteries or some other form of storage.  

String: A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

Tilt angle: The angle at which a photovoltaic array is set to face the sun, relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

Tracking array: A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface.  

Two‐axis tracking: A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).  

Ultraviolet: Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

Volt (V): A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.  

Voltage: The amount of electromotive force, measured in volts, that exists between two points.  

Wafer: A thin sheet of semiconductor (photovoltaic material) made by cutting from a single crystal or ingot.

Zenith angle: The angle between the direction of interest (the sun, for example) and the zenith (directly overhead).