Solar photovoltaic panels, often shortened to solar PV, turn sunlight straight into electricity. The name explains it: "photo" means light, and "voltaic" means electricity. There are no moving parts and nothing is burned.

They are different from solar thermal panels, which use the sun to heat water. Solar PV makes electricity.

What happens inside a panel

A solar panel is made up of many small cells, usually made from silicon. Silicon is the same material used in computer chips.

When sunlight hits a cell, it knocks tiny particles called electrons loose. The cell is built so that those loose electrons can only flow one way. A flow of electrons is an electric current, so the panel is now producing electricity.

This electricity is direct current, or DC. DC flows in one direction, like the power from a battery. Buildings run on alternating current, or AC, which is what comes out of a wall socket.

From panel to plug

A solar system has a few main parts:

The panels, usually wired together in groups called strings.
An inverter, which turns the DC from the panels into AC the building can use.
Protection and switching equipment, so the system can be isolated safely.
A meter or monitoring system, so you can see what the panels are producing.

The building uses solar power first. If the panels make more than the building needs, the extra can be stored in batteries, used to heat water, or sent out to the grid.

Power and energy

Two units come up a lot. A kilowatt, or kW, is the rate at which power is produced or used at any moment. A kilowatt hour, or kWh, is an amount of energy: one kilowatt for one hour. A system rated in kW tells you its size. The kWh it produces tells you how useful it has been over a day or a year.

What affects how much it produces

Brightness of the light. Panels still work on cloudy days, but they produce less.
The season. Long summer days produce far more than short winter ones.
Which way the panels face. In the UK, south-facing panels give the most energy over a year. East- and west-facing panels spread production more evenly from morning to evening, which can suit a building that uses power all day.
Shade. Even partial shade from a chimney or tree can reduce output.
Temperature. Panels work slightly better when they are cool.
Dirt. Panels on a reasonable slope are mostly cleaned by the rain.

How long they last

Despite some rumours, good panels last a long time, pay for themselves quickly and are very safe when installed properly. With no moving parts, there is very little to wear out. The inverter is the part most likely to need replacing during the life of a system.

Solar works best as part of a system

The sun shines in the day, but many buildings also need power at night. That is why we like to install solar alongside battery storage. The panels charge the batteries during the day, and the batteries run the building after dark. Add a heat pump and the same sunshine can heat and cool the building too.