Solar panels explained: from daylight to a lower bill

Solar photovoltaic (PV) panels are the most widely fitted piece of home-energy kit in Britain, and the easiest to misunderstand. This guide takes you from "what is that black rectangle on the roof" to understanding the physics, the numbers and the decisions that separate a good install from a poor one.

The one-sentence version

Solar panels turn daylight into direct-current electricity, an inverter converts it to the alternating current your home uses, and you use that power first, exporting any surplus to the grid for a payment. No daylight, no generation, which is why storage and timing matter so much.

How a solar cell actually works

A panel is a grid of silicon cells. Silicon is a semiconductor: treat it with tiny amounts of other elements (doping) and you create a junction with a built-in electric field. When a photon of light hits the cell, it knocks an electron loose. The field pushes that electron one way, and if you connect a circuit, the electrons flow as current. That flow is the "photovoltaic effect". The brighter the light, the more electrons freed, so output tracks irradiance (the power of sunlight hitting the panel, measured in watts per square metre) rather than temperature or visible "sunniness" alone. Panels still generate on a bright overcast day, just less.

From cell to system: the components

A home system is more than panels. The parts that matter:

• Panels (modules). Rated in watts-peak (Wp) under standard test conditions. Today's panels are typically 400 to 450 Wp each; ten to twelve of them make a common 4 to 5 kWp array. Most are monocrystalline, which is more efficient than the older polycrystalline.
• Inverter. Converts DC to AC. A string inverter wires panels in series; microinverters or power optimisers work per panel and cope better with partial shade. The inverter is the part most likely to need replacing within the system's life.
• Mounting and isolators, plus a generation meter and the wiring that ties it into your consumer unit.

What decides how much you generate

Five things set the yield of a UK array, in rough order of impact:

• Orientation. South is best. East or west loses roughly 15 to 20% over a year but spreads generation across morning and evening, which can suit self-use better.
• Tilt. Around 30 to 40 degrees is close to ideal in the UK; most pitched roofs are already near this.
• Shading. The enemy of solar. Because cells are wired together, shade on one part can drag down a whole string, which is why optimisers or microinverters earn their keep on roofs with a chimney or tree.
• Location and the weather. The south of England gets more annual irradiation than the north of Scotland, but the difference is smaller than people expect.
• System size. A 4 kWp array on a typical UK roof produces about 3,600 to 4,000 kWh a year.

The economics

Solar saves money two ways. First, every unit you generate and use yourself is a unit you do not buy at the import rate (around 26p per kWh under the current price cap). Second, surplus you export earns a payment under the Smart Export Guarantee (SEG), typically a few pence to the mid-teens per kWh depending on supplier. Self-use is worth far more than export, so the value of a system depends heavily on how much of your generation you actually use in the home. A typical 4 kWp install costs in the region of £5,000 to £8,000 fitted and pays back over roughly 7 to 14 years; it benefits from 0% VAT until 31 March 2027. Panels degrade slowly, around 0.5% a year, and keep working for 25 years or more, so most of their life is beyond payback.

Self-consumption: the number that really matters

Generation peaks at midday; home demand peaks morning and evening. Left alone, a typical home uses only 30 to 50% of what its panels make and exports the rest cheaply. The expert move is to raise self-consumption: run the dishwasher and washing machine in the day, add a battery to store the midday surplus for the evening, divert spare power to heat hot water, or charge an EV when the sun is out. Each of those turns low-value export into high-value self-use.

The details an expert checks

Beyond the headline kWp, the things that separate a good install: an inverter sized sensibly to the array (a little undersizing is normal and fine); optimisers where shading exists; an MCS-certified installer (required to claim SEG) and proper DNO notification for the grid connection; roof condition and remaining life, since you do not want to lift panels to re-roof in five years; and a realistic generation estimate based on your actual roof, not a generic figure. Watch for quotes that quietly inflate yield or self-use to shorten the payback.

Common myths

Solar does not need direct sun or a hot day, just light, and panels actually lose a little efficiency when very hot. It works in winter, at lower output. It does not power your home in a blackout unless you have specific battery backup hardware, because standard inverters shut off for safety. And it does not "wear out" suddenly; it fades gently over decades.

Where to go next

Model your own roof's yield, savings and payback with the solar calculator, see whether storage stacks up in are home batteries worth it, and learn how to read a quote in how to read a solar quote. The headline question is answered in is solar worth it in 2026?

Lifespan, maintenance and degradation

Solar is close to fit-and-forget. There are no moving parts in the panels, so there is little to maintain beyond keeping them broadly clear of heavy debris; UK rain handles most cleaning, and steep roofs self-clean well. The slow fade matters more than maintenance: panels lose roughly 0.4 to 0.6% of their output a year, so a 25-year-old array still produces around 85 to 90% of its original yield. Manufacturers reflect this with two warranties worth reading separately: a product warranty (typically 12 to 25 years against faults) and a performance warranty (a guaranteed minimum output, often around 85 to 90% at year 25). The inverter is the weak link, with a shorter life of perhaps 10 to 15 years, so budget for one replacement across the system's life. None of this changes the headline: most of a panel's working life happens after it has paid for itself, which is why solar is best judged over decades rather than a few years.

Bifacial panels, half-cut cells and other jargon

The market moves quickly, and quote sheets are full of terms. Half-cut cells split each cell in two to cut internal losses and cope a little better with shade. PERC and the newer TOPCon and heterojunction cell designs squeeze out more efficiency. Bifacial panels capture some light reflected onto their rear, useful on light-coloured flat roofs but marginal on a typical pitched tile roof. For most homes these differences are second-order next to orientation, shading and a clean install; treat them as tie-breakers between otherwise similar quotes rather than reasons to pay a large premium.