Solar glossary

The terms that turn up on a solar quote and across this site, in plain English. Hover any underlined term in our guides for the short version; this is the long one.

Units & power

PV (photovoltaic): Photovoltaic (PV) means generating electricity directly from light. A solar PV system is the panels, inverter and wiring that turn sunlight into usable power — as opposed to solar thermal, which makes heat.
kWp (kilowatt-peak): Kilowatt-peak (kWp) is the rated power of a solar array under Standard Test Conditions. It describes the size of a system, not what it makes on a given day; real output depends on your location, orientation, tilt and weather.
Wp (watt-peak): Watt-peak (Wp) is the same rating as kWp but for one panel: its output under Standard Test Conditions. A 440 Wp panel is rated at 440 watts, and about 2.3 of them make 1 kWp.
kWh (kilowatt-hour): A kilowatt-hour (kWh) is a unit of energy: one kilowatt of power used for one hour. It is the unit your electricity bill is measured in, and the unit we use for how much a solar system generates.
DC (direct current): Direct current (DC) flows in a single direction. Solar panels generate DC and batteries store it; an inverter converts it to AC for your home and the grid.
AC (alternating current): Alternating current (AC) reverses direction many times a second. It is the form of electricity the grid delivers and household appliances use, so a solar system’s DC output must be inverted to AC before you can use it.
STC (Standard Test Conditions): Standard Test Conditions (STC) are the controlled lab settings — 1,000 W/m² of light, a 25 °C cell temperature and a set air-mass spectrum — under which panels are rated. Real rooftops rarely match them, which is why a system’s actual output differs from its kWp rating.

Cell technology

Monocrystalline: Monocrystalline panels use cells cut from a single silicon crystal, giving higher efficiency (about 20–23%) and a uniform black look. They are the default residential choice in 2026.
Polycrystalline: Polycrystalline (multicrystalline) panels are made from silicon cast in blocks of many crystals. They are cheaper but less efficient (about 15–17%) and have largely disappeared from new residential installs.
Thin-film: Thin-film panels deposit a thin layer of semiconductor (such as CdTe or CIGS) onto a backing. They are lighter and cheaper per panel but less efficient than silicon, so they suit commercial, curved or weight-limited roofs more than homes.
Bifacial: Bifacial panels generate from both faces, picking up light reflected onto their back. The rear gain only matters over a bright surface with an air gap behind the panel, so it helps ground mounts and flat commercial roofs more than dark tiled roofs.
PERC: PERC (Passivated Emitter and Rear Cell) adds a reflective layer behind the cell to capture more light. It was the mainstream silicon cell design through the late 2010s and early 2020s, before TOPCon overtook it.
TOPCon: TOPCon (Tunnel Oxide Passivated Contact) adds a thin oxide layer that cuts electrical losses, lifting module efficiency to roughly 22–24%. By 2026 it had replaced PERC as the mainstream cell technology.
HJT (heterojunction): HJT (heterojunction) cells combine crystalline and thin-film silicon layers. They reach efficiency similar to TOPCon but hold their output better in heat, at a higher price — a premium choice in 2026.
Degradation: Degradation is the gradual loss of output as panels age — a median of roughly 0.5% a year for modern silicon (NREL). It is why a panel is warranted to still produce around 87–90% of its rating after 25 years.

Inverters

Inverter: An inverter converts the direct current (DC) your panels generate into the alternating current (AC) your home and the grid use. It is the most important component after the panels themselves, and usually the first to need replacing.
String inverter: A string inverter converts the DC from a series-wired “string” of panels into AC at a single central unit. It is the cheapest setup, but the whole string runs at the level of its weakest (for example, shaded) panel.
Microinverter: A microinverter is a small inverter fitted to each individual panel, instead of one central (string) inverter for the whole array. Shade or a fault on one panel no longer drags down the rest, and you get per-panel monitoring.
Power optimiser: A power optimiser sits on each panel and conditions its output before sending it to a central string inverter. It captures most of the per-panel benefit of microinverters — shade tolerance and monitoring — while keeping one central inverter.
Hybrid inverter: A hybrid inverter handles both solar panels and a battery in one unit, deciding when to use, store or export power. Fitting one keeps you “battery-ready” without rewiring the system later.
MPPT: MPPT (Maximum Power Point Tracking) is the inverter function that continuously adjusts the electrical operating point of the panels to extract the most power as light and temperature change.

Economics & policy

Payback period: The payback period is how long until cumulative savings cover what you paid upfront. Simple payback ignores the time value of money; discounted payback accounts for it and is the more honest figure.
NPV (Net Present Value): Net Present Value (NPV) totals all the savings a system makes over its life, discounted back to today’s money, minus the upfront cost. A positive NPV means the investment beats simply keeping the cash.
IRR (Internal Rate of Return): Internal Rate of Return (IRR) expresses a solar investment’s lifetime cash flows as a single annual percentage return, so you can compare it with other uses of the money. It ignores the size of the investment, so read it alongside NPV.
VAT: Value Added Tax (VAT) is a sales tax. Several countries (the UK, Germany, the Netherlands) charge 0% VAT on residential solar, which is already baked into the price an installer quotes.
Self-consumption: Self-consumption is the proportion of your solar output you use directly in the home instead of exporting. Because import prices are usually far above export rates, higher self-consumption — often raised with a battery — is where most of the savings come from.
Feed-in tariff: A feed-in tariff pays you a fixed rate for every kilowatt-hour you export to the grid, often guaranteed for years. Many countries have replaced or shrunk these in favour of lower market-linked export rates.
Net metering: Net metering lets the grid credit the electricity you export against what you import, effectively at the retail price. It is generous to solar owners and is being phased out or reduced in several markets.
SEG (Smart Export Guarantee): The Smart Export Guarantee (SEG) is the UK scheme, launched January 2020 and administered by Ofgem, under which larger electricity suppliers must pay small generators for power exported to the grid. It replaced the export part of the old Feed-in Tariff; suppliers set their own rates, which must be above zero.
Salderingsregeling: The salderingsregeling is the Netherlands’ net-metering arrangement, letting households offset exported solar against imported power at the full retail rate. It ends on 1 January 2027, after which a smaller export payment applies.

Measurement & grid

Specific yield: Specific yield is how many kilowatt-hours a system produces per kilowatt-peak installed in a year (kWh/kWp/yr). It captures the local climate and siting, and lets you compare locations independently of system size.
Tilt (roof tilt): Tilt (or roof tilt) is the angle your panels sit at relative to horizontal. Together with the direction they face, it sets how much of the year’s available sunlight they capture; around 30–40° is typical at mid-latitudes.
Azimuth: Azimuth is the compass direction a panel array faces, in degrees. In the northern hemisphere, due south (180°) maximises annual yield; east- or west-facing arrays shift generation toward morning or evening and lower the yearly total slightly.
Capacity factor: Capacity factor is a system’s actual annual output divided by what it would produce running flat-out at its rated power the whole time. Rooftop solar typically lands around 10–20% depending on location.
PVGIS: PVGIS (Photovoltaic Geographical Information System) is the European Commission’s free database of solar irradiation. It estimates how much a system will generate at any set of coordinates and powers the yield figures in our calculator.
DNO (Distribution Network Operator): A Distribution Network Operator (DNO) owns and runs the local electricity network in a region. In the UK you (or your installer) must notify, or get approval from, the DNO to connect a solar system.
LFP (lithium iron phosphate): LFP (lithium iron phosphate) is a lithium-battery chemistry common in home storage. It is valued for long cycle life and thermal safety, at slightly lower energy density than other lithium types.
EPC (Energy Performance Certificate): An Energy Performance Certificate (EPC) rates a property’s energy efficiency from A to G. Solar can lift a home’s EPC band, which some studies link to a higher sale price. (In commercial solar, EPC can also mean Engineering, Procurement & Construction.)
Tier 1: Tier 1 is BloombergNEF’s list of “bankable” module makers — those whose panels have been used in several projects financed by major banks over the past two years. It signals financial acceptance, not product quality; BNEF itself stresses it is not a quality rating.
VPP (Virtual Power Plant): A Virtual Power Plant (VPP) links thousands of home batteries and controls them as one fleet, paying owners to release stored power when the grid needs it. It can add an income stream to a home battery.

Brands

Qcells: Qcells is a major solar manufacturer, German in origin and now owned by South Korea’s Hanwha Group, with factories in the US, Malaysia and South Korea. A widely used mid-market residential brand.
JA Solar: JA Solar is among the largest Chinese solar manufacturers by volume and a mainstream Tier 1 module maker, common in residential and utility projects.
JinkoSolar: JinkoSolar is one of the world’s largest panel manufacturers by shipments, a Chinese Tier 1 maker of mainstream modules.
Trina Solar: Trina Solar is a large Chinese Tier 1 manufacturer, one of the high-volume makers of mainstream residential and commercial panels.
LONGi: LONGi is the world’s largest solar manufacturer by capacity, a Chinese Tier 1 maker known for monocrystalline modules.
Canadian Solar: Canadian Solar is a large manufacturer headquartered in Canada but producing mainly in China and Asia; a mainstream Tier 1 brand.
REC: REC Group is a premium panel brand founded in Norway and manufacturing in Singapore, known for higher-quality residential modules.
Maxeon: Maxeon makes premium high-efficiency panels carrying SunPower’s cell technology (spun out in 2020). It is now majority-owned by China’s TCL/Zhonghuan, which rescued it in 2024.
Aiko: Aiko is a Chinese manufacturer known for high-efficiency back-contact (ABC) residential panels — a newer premium option.
Panasonic: Panasonic was a long-standing premium Japanese solar brand, but it exited solar-panel manufacturing in 2025 — a reminder that a 25-year warranty is only as good as the company behind it.
Enphase: Enphase is a US company that pioneered the microinverter and remains the leading microinverter brand for residential solar.
SMA: SMA is a German manufacturer of string and hybrid solar inverters, long established in residential and commercial solar.
Fronius: Fronius is an Austrian manufacturer of string and hybrid solar inverters, widely used in residential installs.
SolarEdge: SolarEdge is an Israeli company known for power optimisers used alongside its string inverters, a common alternative to microinverters.