Solar glossary

How to use this glossary

Solar has its own language, yet most of it is simple once someone explains it. We group the terms by topic so you can read a whole section at once. Each entry links to a guide when you want more detail.

The sources at the foot of the page back the technical and legal definitions. Where a rule applies to one nation, we say so. We reviewed the page in October 2026.

Units and measures

kW (kilowatt)

A kilowatt is a measure of power at one moment. One kilowatt is 1,000 watts. Because it is a rate, a kW figure says nothing about how much energy you get over a day.

kWh (kilowatt-hour)

A kilowatt-hour is the energy a 1kW device uses in one hour. It is the unit on your electricity bill, so it is the one to watch. Your panels, your battery and your appliances are all counted in kWh over time.

kWp (kilowatt peak)

kWp is the rated peak output of a solar array under standard test conditions. MCS adds up the label values of every panel to get it. Real output is usually lower than the peak figure, because real light and heat rarely match the test.

Generation

Generation is the electricity your panels produce. It is counted in kWh. Not all of it reaches your appliances, since some is lost in the wiring and the inverter, and some is exported.

Performance estimate

This is the installer’s forecast of how much your system will produce in a year. MCS says it is worked out as kWp multiplied by a location factor and a shading factor. The installer must give it to you before you sign, and it is guidance for the first year only. It is not a guarantee. See how many solar panels you need for how size and estimate link up.

Direct current and alternating current (DC and AC)

Panels make direct current, which flows one way. Your home and the grid use alternating current, which changes direction many times a second. Since the two do not match, the inverter converts one into the other.

Degradation

Degradation is the slow fall in a panel’s output as it ages. It is gradual, yet it adds up over decades. Read solar panel lifespan and degradation for how to read a performance warranty.

Payback period

Payback is the time it takes for your savings and income to repay what you spent. No single figure suits every home, because it depends on your tariff, your use and the price you paid. Ask any installer to show their working, and see the costs and savings hub for how to build your own sums.

Parts of a system

Solar panel (module)

A solar panel is a flat unit that turns daylight into electricity. The MCS standard calls each one a module. Your quote should name the make and model of the panels.

Array

An array is the whole group of panels on your roof. MCS rates it in kWp. While the array looks like one block, it is wired so that it feeds one or more inverters.

Inverter

The inverter converts the direct current from the panels into alternating current for your home. It also manages the link to the grid. It works hard, so it is often the first part to need a replacement. See solar inverters explained and the maintenance and lifespan hub.

String inverter

A string inverter is a single box that handles a whole row, or string, of panels. It is a common choice. However, shade on one panel can pull down the output of the whole string.

Microinverter

A microinverter is a small inverter fitted behind each panel. Each panel then works on its own. This can help on roofs with shade or several faces, although it adds more parts on the roof.

Power optimiser

An optimiser is a small device at each panel that tunes its output. MCS calls it a module level power electronics device. If your quote includes one, the installer must give you its datasheet.

Battery (energy storage)

A solar battery stores spare electricity for later use, for example in the evening. MCS uses the term electrical energy storage system, or EESS. See solar battery cost and sizing and the batteries and export hub.

Mounting system

The mounting system is the rails, clamps and brackets that hold the panels to the roof. It must stay secure and must not harm the roof. MCS asks installers to check clamps and rails at a full maintenance visit.

In-roof system

An in-roof system sits in place of the normal tiles and forms part of the roof covering. It looks neater than panels raised above the tiles. However, MCS says it needs a kit with the right fire rating, so ask your installer for the proof.

Isolator

An isolator is a safety switch that cuts the electrical link between parts of the system. Your system will have one on the DC side and one on the AC side. Know where they are, because an electrician or engineer may need them.

Monitoring app

A monitoring app shows what your system produces, often by the hour. It helps you spot faults early, because a drop in output is easy to see. Our guide to reading your solar monitoring app explains the graphs.

Solar diverter

A diverter sends spare solar power to a hot water cylinder instead of exporting it. The water then holds that energy as heat. See solar diverters for hot water.

Roof and site

Orientation

Orientation is the compass direction your panels face. In the UK, a south-facing roof tends to give the most yearly output. East and west roofs can still work well, although they suit a different pattern of use.

Pitch

Pitch is the angle of the roof. The MCS estimate uses pitch, orientation and shade together. Therefore a small change in any one of them changes the forecast.

Shading and shade factor

Shading is anything that blocks the sun from the panels, such as trees, chimneys or nearby buildings. MCS turns shading into a shade factor for the estimate. A factor below 1 means shade is present, and the estimate must then carry a shading note.

East-west system

An east-west system puts panels on two opposite roof faces. It produces more in the morning and evening and less at noon, so the day’s output is spread more evenly. See east-west and flat roof solar and the roof and property suitability hub.

Flat roof system

A flat roof system uses frames to tilt panels at a set angle. It gives the designer more freedom over direction. However, the planning rules differ from pitched roofs, so check the planning guidance.

Permitted development

Permitted development lets you carry out some work without applying for planning permission. In England, the Planning Portal says solar rules changed on 27 August 2026, with a transition until 27 August 2027. In Wales, Welsh Government says panels should not project more than 200mm from the roof or wall surface.

Conservation area and listed building

In a conservation area, England’s rules say panels must not go on a wall that faces a road. The Planning Portal says permitted development does not cover buildings in the grounds of a listed building. In Wales, listed buildings need planning permission and listed building consent.

Article 4 direction

An Article 4 direction is a council order that removes some permitted development rights in an area. If one applies to your street, you will need formal planning permission. Welsh Government advises homeowners to check for one.

Building regulations and competent person schemes

Building regulations set safety standards for building work, including electrics. In England, a firm on a competent person scheme can certify its own electrical work. As a result, you do not need to involve building control separately. See planning your installation.

Money and tariffs

Smart Export Guarantee (SEG)

The SEG makes licensed suppliers pay small generators for electricity they export. It began on 1 January 2020 and covers solar up to 5MW. Each supplier sets its own rates, but the rate must be above zero. See the Smart Export Guarantee explained.

SEG licensee

A SEG licensee is a supplier that offers an export tariff. Suppliers with at least 150,000 domestic customers must offer one. Smaller suppliers may join voluntarily, and you need not use your own supplier for exports, since the two can differ.

Export tariff

An export tariff is the rate you are paid per kWh sent to the grid. Rates differ between suppliers and change over time. For that reason, we name the supplier and the date whenever we quote one.

Feed-in Tariff (FIT)

The Feed-in Tariff was the earlier scheme that paid for both generation and export. It closed to new applicants on 31 March 2019. The SEG followed it, but works very differently.

Self-consumption

Self-consumption is the share of your solar power that you use at home. Every kWh you use yourself saves you the price of a kWh from the grid. See how to use more of your own solar power.

Export and import

Export is power sent from your home to the grid. Import is power drawn from the grid. A home with solar often does both in the same day, because output rises and falls with the light.

Time-of-use tariff

A time-of-use tariff charges different prices at different times of day. It rewards you for using power when it is cheap. With a battery, you can also store cheap power for later. Read time-of-use tariffs with solar and batteries.

Zero rate VAT

HMRC says solar panels and batteries installed in homes carry a zero rate of VAT until 31 March 2027. From 1 April 2027 the rate returns to 5%. See 0% VAT on solar panels.

Warm Homes: Local Grant

This is a government scheme that pays for energy-saving work for eligible homes, such as low-income households. GOV.UK says it covers England only and lists solar panels among possible measures. See solar panel grants and funding and free solar panel schemes explained.

Grid and connection

Distribution network operator (DNO)

The DNO runs the local electricity network in your area. It is not the same company as your supplier. The MCS standard says your installer must notify the DNO about your system, so you should not need to do it yourself.

G98

G98 is the connection rule for small, type-tested generators. MCS says it applies to systems up to and including 16 amps per phase. The 16 amp limit counts all generators together, so see the next entry.

G99

G99 is the connection rule for larger systems. It applies above 16 amps per phase in total. For example, a 3kW solar system plus a 3kW battery on one single-phase supply goes over the limit, so G99 applies. See connecting solar to the grid.

MPAN

An MPAN is a 13-digit number that identifies an electricity connection point. You have an import MPAN on your bill. To get SEG payments you also need an export MPAN, which your SEG licensee can give you.

Export meter

An export meter measures what you send to the grid. Ofgem says it must be able to take half-hourly readings. Your existing meter might already do this, so ask your SEG licensee.

Standards and protection

MCS (Microgeneration Certification Scheme)

MCS is the quality scheme for small renewable systems in the UK. It certifies installers and sets standards for installs, because the certificate often decides what support you can claim. See MCS certification explained.

MIS 3002

MIS 3002 is the MCS installation standard for solar PV. Issue 6.0 is dated 18 March 2026. It covers design, install, handover and advice on upkeep for systems up to 50kWp.

MCS certificate

The MCS certificate proves that your system was registered after install. The installer registers it within 10 working days of commissioning. Keep it, because it is often the proof a supplier asks for before SEG payments.

RECC

RECC is the Renewable Energy Consumer Code. It sets sales rules for its members, including deposits, contracts and complaints. See choosing a solar installer.

Deposit

A deposit is money paid before the work starts. RECC tells homeowners never to pay more than 25% of the contract price up front. It also says the deposit should be insured.

Insurance-backed guarantee

This is an insurance policy that backs your installer’s guarantee. It pays out if the installer stops trading. MCS says it is mandatory for domestic installs and must cover workmanship for at least two years.

Workmanship guarantee

A workmanship guarantee covers the quality of the install. It is given by the installer. It protects you against faults such as poor wiring or leaking roof fixings, while the maker covers the parts themselves.

Product warranty

A product warranty is the maker’s promise on a part, such as a panel or an inverter. It is separate from the installer’s guarantee. Check how long each one runs, because inverter terms are often shorter.

Performance warranty

A performance warranty is the maker’s promise about how much output panels will still give after a set time. It is one of the main papers to keep. Ask for the exact terms in writing.

Cancellation period

This is the time you can change your mind after you sign. For contracts signed at home or at a distance, it is normally 14 days, but the exact rules depend on how you sign. The installer needs your written consent to start inside that window.

Handover pack

The handover pack is the set of papers you get when the system is finished. MCS says it must include the user manuals, warranties, a signed declaration and a suggested date for the first inspection. Keep it with your house papers.

Section 75

Section 75 of the Consumer Credit Act 1974 can make a card provider share liability with the seller. It applies to a single item with a cash price over £100 and up to £30,000. Ask your card provider how it treats solar work.

Keep exploring

If a term here is new to you, start with how solar panels work. Then read the guide that matches your next step. Our sources and methodology page shows how we check the facts above. You can also read solar panels in winter to see how several of these terms play out through the year.