Every term you are likely to meet in a solar quote, a battery spec sheet, a utility rule or a research paper, in plain language. Type in the box to search, or filter by topic. Missing a term? Suggest it in the discussion at the bottom.
A
- AgrivoltaicsFarming and land
- Using the same land for solar panels and farming, such as growing crops beneath raised panels or grazing sheep among them.
- AlbedoSolar PV
- How much sunlight a surface reflects. Light colored ground or snow raises albedo and boosts output from bifacial panels.
- Alkaline electrolyzerHydrogen
- A mature, lower cost type of electrolyzer that uses a liquid potassium hydroxide solution to split water into hydrogen and oxygen.
- Alternating current (AC)Grid and utility
- Electricity that reverses direction many times per second (60 times in most of North America, 50 or 60 elsewhere in the Americas). Homes and the grid use AC.
- Ammonia (green)Hydrogen
- Ammonia made from hydrogen produced with renewable electricity and nitrogen from the air. It can be used as fertilizer, fuel or a way to store hydrogen.
- Amp (ampere)Installation and safety
- The unit of electric current, the rate at which charge flows through a wire.
- Amp hour (Ah)Storage
- A battery capacity rating: the current a battery can deliver for one hour. Multiply by voltage to get watt hours.
- Anti islandingGrid and utility
- A safety feature that makes grid tied inverters shut off during a blackout so they do not energize lines that utility crews are repairing.
- ArraySolar PV
- A group of solar panels wired together to work as one system.
- AzimuthInstallation and safety
- The compass direction a panel faces. In the Northern Hemisphere south facing is usually best; in the Southern Hemisphere, north facing.
B
- BackfeedGrid and utility
- Power flowing from your system back into the grid. Also a safety risk if a generator is connected without a transfer switch.
- Balance of system (BOS)Solar PV
- Everything in a solar system except the panels: racking, wiring, inverters, disconnects and monitoring.
- Battery management system (BMS)Storage
- Electronics inside a battery that monitor cell voltage, temperature and current to keep it safe and balanced.
- Behind the meterGrid and utility
- Equipment on the customer side of the utility meter, such as a rooftop system or home battery.
- Bifacial panelSolar PV
- A panel that collects light on both front and back, gaining extra output from light reflected off the ground.
- BusbarSolar PV
- Thin metal strips on a solar cell that collect current. Also the metal bar in an electrical panel that distributes power to breakers.
C
- C rateStorage
- How fast a battery charges or discharges relative to its capacity. 1C empties a full battery in one hour; 0.5C takes two hours.
- Capacity factorGrid and utility
- Actual energy produced over a year divided by what a plant would make running at full power all the time. Solar is typically 15 to 30 percent.
- Carbon intensityMoney and policy
- Grams of carbon dioxide equivalent emitted per kWh of electricity over a technology's full life cycle.
- Charge controllerStorage
- A device between panels and batteries in off grid systems that prevents overcharging. MPPT controllers are the most efficient type.
- Combiner boxInstallation and safety
- An enclosure where several strings of panels are joined, usually with fuses, before running to the inverter.
- Community solarMoney and policy
- A shared solar farm where households subscribe to a portion and receive credits on their bills without panels on their own roofs.
- Compressed air energy storage (CAES)Storage
- Storing energy by pumping air into caverns or tanks under pressure, then releasing it through a turbine.
- Crystalline siliconSolar PV
- The material in over 95 percent of solar panels sold today, either monocrystalline or polycrystalline.
- CurtailmentGrid and utility
- When a grid operator tells generators to cut output because there is more supply than demand or not enough transmission.
- Cycle lifeStorage
- How many full charge and discharge cycles a battery can deliver before its capacity falls to a set level, often 70 or 80 percent.
D
- Degradation rateSolar PV
- How much output a panel loses each year. Quality panels lose around 0.3 to 0.5 percent per year.
- Demand chargeMoney and policy
- A fee on commercial bills based on the highest power drawn in a billing period. Batteries can reduce it by trimming peaks.
- Depth of discharge (DoD)Storage
- The share of a battery's capacity that is used. Using less of the capacity per cycle usually extends battery life.
- Direct current (DC)Solar PV
- Electricity that flows in one direction. Solar panels and batteries produce DC.
- Direct pay (elective pay)Money and policy
- A U.S. rule letting tax exempt groups, tribes and public bodies receive certain clean energy credits as a cash payment.
- Distributed generation (DG)Grid and utility
- Electricity produced close to where it is used, such as rooftop solar, instead of at large central plants.
E
- ElectrolyzerHydrogen
- A device that uses electricity to split water into hydrogen and oxygen.
- Energy (kWh)Grid and utility
- Power used over time. One kilowatt running for one hour uses one kilowatt hour, the unit on most electric bills.
- Energy arbitrageStorage
- Charging a battery when electricity is cheap and discharging when it is expensive.
- Energy payback timeMoney and policy
- How long a solar panel must operate to generate the energy that was used to make it. For modern panels it is roughly one to three years.
- Export limitGrid and utility
- A cap on how much power a system may send to the grid, set by the utility or by inverter settings.
F
- Feed in tariff (FiT)Money and policy
- A fixed price paid for every kWh of renewable electricity sent to the grid, usually under a long term contract.
- Fill factorSolar PV
- A measure of solar cell quality comparing its real maximum power to the theoretical product of its open circuit voltage and short circuit current.
- Floating solar (floatovoltaics)Solar PV
- Solar panels mounted on floats on reservoirs, ponds or lakes. Water cools the panels and the panels reduce evaporation.
- Flow batteryStorage
- A battery that stores energy in liquid electrolytes held in tanks. Capacity grows by adding more liquid, which suits long duration storage.
- FlywheelStorage
- A spinning rotor that stores energy as motion. Very fast response for grid stability, but usually only minutes of storage.
- Fuel cellHydrogen
- A device that combines hydrogen and oxygen to make electricity, heat and water with no combustion.
G
- Grid following inverterGrid and utility
- An inverter that synchronizes to an existing grid signal. Most residential inverters work this way.
- Grid forming inverterGrid and utility
- An inverter that can set grid voltage and frequency on its own, allowing solar and batteries to run a microgrid or support a weak grid.
- Grid parityMoney and policy
- The point where solar electricity costs the same as or less than buying power from the grid.
- Ground faultInstallation and safety
- An unintended path for current to reach ground. Inverters and breakers detect it to prevent shocks and fires.
- Ground mountInstallation and safety
- A solar array fixed to a frame on the ground instead of a roof.
H
- Heterojunction (HJT)Solar PV
- A high efficiency cell design that layers thin amorphous silicon on crystalline silicon.
- Hybrid inverterStorage
- An inverter that manages solar panels, a battery and the grid in one unit.
- Hydrogen fuel cell vehicleHydrogen
- An electric vehicle that makes its own electricity from hydrogen in a fuel cell instead of storing it all in a battery.
I
- Interconnection agreementGrid and utility
- The contract with your utility that allows a system to connect to the grid and sets the technical and safety rules.
- InverterSolar PV
- The device that turns DC electricity from panels or batteries into AC electricity for your home and the grid.
- IrradianceSolar PV
- The power of sunlight striking a surface, measured in watts per square meter. Full sun is about 1,000 W/m².
- Irradiation (insolation)Solar PV
- The total solar energy received over time, usually in kWh per square meter per day.
- IslandingGrid and utility
- When part of the grid, or a building with solar and batteries, keeps running while disconnected from the main grid.
J
- Junction boxSolar PV
- The small sealed box on the back of a panel where cells connect to the output cables, often with bypass diodes.
K
- Kilowatt (kW)Grid and utility
- A unit of power equal to 1,000 watts. Residential systems are usually 4 to 15 kW.
- Kilowatt peak (kWp)Solar PV
- The rated output of solar panels under standard test conditions.
L
- LCOE (levelized cost of energy)Money and policy
- The average cost of each kWh a power plant produces over its life, including building, fuel, operations and financing.
- LCOS (levelized cost of storage)Money and policy
- The average cost of each kWh delivered by a storage system over its life.
- LFP (lithium iron phosphate)Storage
- A lithium battery chemistry with long life and strong thermal stability, now the most common choice for home and grid storage.
- Lithium ion batteryStorage
- A family of rechargeable batteries that move lithium ions between electrodes. Includes LFP and NMC chemistries.
- LoadGrid and utility
- Anything that uses electricity, or the total demand at a moment.
- Load shiftingStorage
- Moving electricity use from one time to another, often by storing solar for the evening.
M
- Main panel upgradeInstallation and safety
- Replacing a home's electrical panel with a larger one, sometimes needed before adding solar, batteries or EV chargers.
- Metal hydride storageHydrogen
- Storing hydrogen inside metal alloys that absorb it like a sponge, at low pressure and with high safety.
- MicrogridGrid and utility
- A local grid, such as a campus or village, that can run connected to the main grid or on its own.
- MicroinverterSolar PV
- A small inverter mounted under each panel, so each panel works independently and shade on one does not drag down the rest.
- ModuleSolar PV
- The technical word for a solar panel: a framed, sealed group of cells.
- MonocrystallineSolar PV
- Cells cut from a single silicon crystal. Higher efficiency and a uniform dark look.
- MPPT (maximum power point tracking)Solar PV
- Electronics in inverters and charge controllers that constantly adjust voltage to pull the most power from panels.
N
- Net billingMoney and policy
- A system where imports are billed at retail rates but exports are credited at a lower rate, such as the avoided cost of power.
- Net meteringMoney and policy
- A billing arrangement where exported solar energy spins the meter backward, so each exported kWh offsets one bought later.
- Net zeroMoney and policy
- Producing as much energy as you use over a year, or removing as much carbon as you emit.
- NMC (nickel manganese cobalt)Storage
- A lithium battery chemistry with high energy density, common in electric vehicles.
- NOCTSolar PV
- Nominal operating cell temperature: a rating that describes panel output under more realistic heat than standard test conditions.
O
- Off gridStorage
- A system with no connection to the utility grid, relying on solar, batteries and often a backup generator.
- Open circuit voltage (Voc)Solar PV
- The voltage of a panel when nothing is connected. Used to size strings safely, especially in cold weather when voltage rises.
- Operations and maintenance (O&M)Installation and safety
- Cleaning, inspections, repairs and monitoring that keep a system producing.
P
- Payback periodMoney and policy
- How long it takes for savings to add up to the system cost.
- Peak shavingStorage
- Using a battery to cut the highest points of demand, lowering demand charges or grid stress.
- Peak sun hoursSolar PV
- The number of hours per day when sunlight would equal 1,000 W/m² if concentrated. A shortcut for estimating output.
- PEM electrolyzerHydrogen
- Proton exchange membrane electrolyzer: compact and quick to respond to changing solar output, but uses platinum group metals.
- Performance ratioSolar PV
- Actual output divided by theoretical output, capturing losses from heat, wiring, dust and inverters. Typically 75 to 85 percent.
- Permission to operate (PTO)Grid and utility
- The utility's approval to switch on a grid connected system after inspection.
- PerovskiteSolar PV
- A family of crystal materials that can make efficient, potentially cheap solar cells. Durability is the main challenge.
- Photovoltaic (PV)Solar PV
- Converting light directly into electricity using semiconductors.
- PolycrystallineSolar PV
- Cells made from many silicon crystals melted together. Cheaper but less efficient; now largely replaced by monocrystalline.
- Power (kW)Grid and utility
- The rate of energy flow at a moment, like the speed of a car. Energy (kWh) is the distance traveled.
- Power optimizerSolar PV
- A device on each panel that tunes its output before sending it to a central string inverter.
- Power purchase agreement (PPA)Money and policy
- A contract to buy electricity from a system someone else owns, often on your roof, at a set price per kWh.
- Pumped hydro storageStorage
- Pumping water uphill to a reservoir when power is plentiful, then releasing it through turbines. The largest form of grid storage worldwide.
R
- RackingInstallation and safety
- The mounting structure that holds panels to a roof or the ground.
- Rapid shutdownInstallation and safety
- A code requirement in many places that de energizes rooftop wiring quickly so firefighters can work safely.
- Renewable energy certificate (REC)Money and policy
- A certificate representing the environmental benefits of 1 MWh of renewable electricity. Solar RECs are called SRECs.
- Round trip efficiencyStorage
- Energy you get out of storage divided by energy you put in. Lithium batteries are about 85 to 95 percent; hydrogen is about 30 to 40 percent.
S
- Self consumptionGrid and utility
- Using your own solar power on site instead of exporting it.
- Shading lossSolar PV
- Lost output from shade. Even partial shade on one cell can cut a whole string unless panels use optimizers or microinverters.
- Short circuit current (Isc)Solar PV
- The current a panel produces when its terminals are connected directly. Used to size wires and fuses.
- Sodium ion batteryStorage
- A battery that uses abundant sodium instead of lithium. Lower energy density but cheaper materials and good cold weather performance.
- SoilingSolar PV
- Dust, pollen, bird droppings or snow on panels that reduce output.
- Solar canopy (carport)Solar PV
- A raised solar structure over parking, walkways or fields that provides shade and power.
- Solar leaseMoney and policy
- Renting a solar system for a monthly fee. The leasing company owns and maintains it.
- Solar trackerSolar PV
- A mount that turns panels to follow the sun, adding 15 to 30 percent output on ground systems.
- Solar water pumpFarming and land
- A pump powered directly by solar panels, widely used for livestock water and irrigation far from the grid.
- Standard test conditions (STC)Solar PV
- The lab conditions used to rate panels: 1,000 W/m² of light, 25°C cell temperature and a set light spectrum.
- State of charge (SoC)Storage
- How full a battery is, as a percentage.
- State of health (SoH)Storage
- A battery's remaining capacity compared with when it was new.
- StringSolar PV
- Panels wired in series so their voltages add up.
- String inverterSolar PV
- A central inverter that serves one or more strings of panels.
T
- Tandem cellSolar PV
- A solar cell that stacks two materials, such as perovskite on silicon, to capture more of the light spectrum.
- Temperature coefficientSolar PV
- How much a panel's output drops for each degree above 25°C. Lower numbers are better in hot climates.
- Thermal energy storageStorage
- Storing heat or cold, in water tanks, molten salt, rocks or ice, for later use.
- Thermal runawayStorage
- A chain reaction of overheating inside a battery cell that can lead to fire. Good chemistry, BMS design and installation reduce the risk.
- Thin filmSolar PV
- Solar cells made from very thin layers of material such as cadmium telluride, used in many utility scale plants.
- Third party ownership (TPO)Money and policy
- Solar that a company owns on your property through a lease or PPA.
- Tilt angleInstallation and safety
- The angle of panels from horizontal. A tilt near your latitude maximizes yearly output.
- Time of use rates (TOU)Money and policy
- Electricity prices that change by time of day. Batteries can save money by avoiding expensive evening hours.
- TOPConSolar PV
- Tunnel oxide passivated contact: a high efficiency cell design that became the mainstream panel technology in the mid 2020s.
- Transfer switchInstallation and safety
- A switch that safely moves a home's circuits between the grid and a backup source.
U
- Utility scaleGrid and utility
- Large solar or storage plants, usually several megawatts or more, selling power to the grid.
V
- Vehicle to grid (V2G)Storage
- Using an electric vehicle battery to send power back to a home or the grid.
- Virtual power plant (VPP)Grid and utility
- Many home batteries, thermostats and EVs managed together by software to act like one power plant.
- VoltInstallation and safety
- The unit of electrical pressure that pushes current through a circuit.
W
- WaferSolar PV
- A thin slice of silicon that becomes a solar cell.
- WattGrid and utility
- The basic unit of power. 1,000 watts is a kilowatt, 1,000 kilowatts is a megawatt.
- Watt hour (Wh)Storage
- A unit of energy. Battery capacity is often given in kilowatt hours.
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