How Do Solar Panels Work?

Solar cells absorb light and produce a direct current, and an inverter converts that into the alternating current your appliances need. The Energy Saving Trust sets it out in four steps on its solar panels advice page, last updated 21 July 2026: sunlight is absorbed by cells made from layers of semi-conducting material, most commonly silicon; the cells produce an electrical charge as they become energised; that charge creates a direct current; the direct current passes through a solar inverter to become alternating current.

That is the whole mechanism. The part worth your attention is not the physics, it is what happens either side of it: how a lab rating on a panel label turns into a kWh figure on your quote, and where the electricity goes once it is alternating current. Both are where money is won and lost. Every figure here was checked against the Energy Saving Trust, the MCS installation standard, Ofgem or government statistics on the day this was written.

What the sunlight actually does inside the panel

Light knocks the silicon into producing an electrical charge, and the brighter the light, the more charge you get. There is no fuel, no combustion and nothing that has to spin. That is why a solar array has so little to go wrong compared with almost anything else in a house.

The Energy Saving Trust makes two points here that people routinely get backwards. First, "the stronger the sunshine, the more electricity generated", so output tracks light intensity rather than temperature. Second, and this is the one that surprises people, cells "don't need direct sunlight to work and can even work on cloudy days". On its solar power fact check page, updated 16 July 2026, it explains why: panels absorb energy from the light spectrum we can see and from "wavelengths that can pass through clouds".

An overcast Tuesday in Salford still generates. It generates less. If you want the seasonal version of that answer with government load factor data behind it, our guide to whether solar panels work in winter has the quarterly numbers.

What kWp means, and why it is not what your roof will produce

kWp is a laboratory rating, not a forecast. The Energy Saving Trust defines it plainly: "The kWp is the maximum amount of power the system can generate in ideal conditions." Ideal conditions are a test bench, not a Wednesday in Coventry.

This is the single most useful thing to understand about a solar quote, because kWp is the number salespeople lead with and kWh is the number that pays you back. The bridge between them is set by the MCS standard every certified installer works to. MIS 3002 Issue 5.0, the Solar PV Standard, gives one formula in Appendix B:

Each term is doing a job:

• kWp is, in the standard's own words, "the sum of the data plate value (Wp at STC) of all modules installed (the value printed on the module label)". It is arithmetic on a label. Nothing about your house is in it yet.
• Kk is a kWh per kWp figure looked up from location-specific tables published on the MCS website, chosen by your postcode zone, your roof pitch and the direction the array faces. This is where the UK actually enters the calculation. Across the country it lands broadly in the 800 to 900 kWh per kWp range, better in the south, lower in the far north.
• SF is the shade factor, worked out from objects blocking the horizon. A neighbour's sycamore lives in this number.

So a 4.5 kWp array at a Kk of 850 with no shading is estimated at roughly 3,800 kWh a year. The same panels behind a chimney are not.

MIS 3002 also requires your installer to hand you that estimate before you sign, in a set format, with a warning attached word for word. Read it. It is blunter than anything you will get in a sales pitch: "The performance of solar PV systems is impossible to predict with certainty due to the variability in the amount of solar radiation (sunlight) from location to location and from year to year. This estimate is based upon the standard MCS procedure is given as guidance only for the first year of generation. It should not be considered as a guarantee of performance."

If shading is present, clause 4.1.4 requires a second note saying the shade factor was calculated using the MCS methodology and that this "will yield results within 10% of the actual energy estimate stated for most systems". An installer who will not show you these numbers is skipping a requirement of the standard they claim to work to. Our guide to choosing an MCS-certified installer covers what else has to be in writing, and roof orientation covers how much the direction of your roof moves the Kk figure.

Where the electricity goes once it is alternating current

It powers whatever is switched on in your house at that moment, and only the leftovers go to the grid. Solar electricity does not queue up politely waiting for you. It takes the shortest path to whatever is drawing current, so the fridge, the router and the immersion heater get served before anything reaches the meter.

That order matters more than any other detail on this page, because the two destinations are worth very different amounts:

• Used in the house: worth whatever you would otherwise have paid for that unit. Under the Ofgem price cap for 1 July to 30 September 2026, a typical direct debit customer in England, Scotland and Wales pays 26.11p per kWh with a 57.19p daily standing charge, giving a cap of £1,862 a year.
• Exported to the grid: worth whatever your supplier pays under the Smart Export Guarantee. The Energy Saving Trust says you "typically get around 12p for every unit that you don't use yourself", and points out you would normally buy electricity for more than twice that.

A unit you use is worth roughly double a unit you sell. That is the whole argument for timing your washing machine, and the whole argument for a battery. Export rates are set by individual suppliers rather than by government and they move, so check the current spread on our Smart Export Guarantee rates page rather than assuming 12p. One geographic point that catches people out: the Smart Export Guarantee covers England, Scotland and Wales only. If you are in Northern Ireland, the Energy Saving Trust advises speaking to your supplier about what export tariff they offer.

Storage changes the timing rather than the total. A battery holds the midday surplus so you can use it at seven in the evening at 26.11p instead of selling it at around 12p, which is covered in full in our solar battery storage guide.

Why your solar panels stop working in a power cut

Because a grid-connected system is designed to shut down when the grid goes down, and that is deliberate. It catches out a lot of new solar owners, and it is better learned before you buy than during a February storm.

The Energy Saving Trust states it directly: "Typically solar panels can't power your home during a power cut. However, they can if your home is off the grid or your system has been installed with additional equipment." It adds that "a battery storage system also does not normally provide power in a power cut unless additional equipment and configuration is included in the installation."

The reason is safety. Your inverter is pushing electricity out onto the same wires an engineer may be working on. Grid-connected generation in the UK is installed under the Electricity Safety, Quality and Continuity Regulations 2002, and MIS 3002 clause 5.1.8 requires MCS contractors to follow the technical requirements of the industry Engineering Recommendations under Regulation 22(2)(c). Those rules are what keep a house from energising a dead street.

Backup during an outage is a specification and a cost, not a default. Ask for it by name at quote stage if you want it.

The grid paperwork, and the trap in it

Your installer notifies the network operator, and which form they use depends on your total output, not just on the panels. MIS 3002 clause 5.1.8 sets the split out precisely:

• EREC G98 for installations up to and including 16 A per phase
• EREC G99 for installations exceeding 16 A per phase
• EREC G100 where the export of power is to be limited

Here is the part that catches households adding storage later. The standard is explicit that "the threshold is the total aggregated AC output of all generators", and it gives the example itself: a 3kW solar PV system and a 3kW battery storage system connected in parallel to the same single-phase supply gives a combined maximum theoretical output greater than 16 A, and in that case G99 applies. Panels that sat comfortably under the G98 threshold on their own can push you over it the day a battery arrives. Our guide to G98 and G99 walks through what each process actually involves.

Either way the paperwork is normally your installer's job. The Energy Saving Trust confirms you must register the system with your Distribution Network Operator and that "usually, your installer will register the device with the DNO for you". In Northern Ireland, it is registered with NIE Networks.

What a typical UK solar system actually looks like

Smaller than the benchmark figure everyone quotes. The Department for Energy Security and Net Zero publishes monthly solar deployment statistics, and the numbers give a reality check that no brochure will.

From Energy Trends table 6.3, published 30 July 2026 with data to the end of June 2026, the UK had 2,075,804 solar PV installations, of which 1,721,751 are recorded as being on domestic properties. Broken down by size, the two smallest bands, which is where household rooftops sit, look like this:

The average sub-10kW installation in this country is about 3.4 kW, and 65% of every solar installation in the UK, of any size, sits in the smallest band under 4 kW. Set that against the Energy Saving Trust's benchmark, which is that "domestic solar panel systems are around 4.5 kWp and cost around £7,600", covering 20 to 30 square metres of roof with around 12 panels. The benchmark describes a good modern install. The installed stock is meaningfully smaller than it, largely because a great many of those systems went up under the old Feed-in Tariff era when 4 kW was the sweet spot.

Two caveats, because the figures deserve them. The capacity bands are not split into domestic and non-domestic, so these averages cover every installation in those size bands rather than houses alone. And DESNZ notes that cumulative capacity figures are revised as it receives data, and that it "may not be notified of new capacity until several months after installation", so the most recent months will fill in over time.

The practical use of that table is as a sanity check. If you are being quoted something far larger than your roof or your consumption justifies, you are being sold capacity rather than savings. How many solar panels you need works from your annual kWh backwards, and cost by system size compares 3 kW to 6 kW side by side.

How you know it is still working

A solar system fails quietly, so the monitoring is not a gadget, it is the fault alarm. Panels have no moving parts and make no noise. A dead string or a failed inverter looks exactly like a cloudy fortnight unless something is telling you otherwise.

MIS 3002 covers this in clause 5.7. A means of recording and displaying the total AC generation of the system "shall be installed", which is a requirement rather than an option. Clause 5.7.5 goes further and says a means of notifying you within 48 hours of the system "ceasing to operate, or beginning to operate abnormally" should be provided. Ask which of these your quote includes and how you will actually see the alert.

On lifespan, the Energy Saving Trust is consistent: "Your solar panels should last 25 years or more", but "if you have a solar inverter, you need to replace this after around 12 years". The panels outlive the box on the wall, which is the single fact most payback calculations forget. Our guide to how long solar panels last goes through the warranty side, and what solar panels cost in the UK covers the whole price picture including that mid-life replacement.