Rooftop solar photovoltaic (PV) panels turn daylight into electricity, and in the UK the sums can work even though the climate is grey. What matters is not how sunny a given day is but how much of the year's generation you use yourself, what you pay for the system, and what you are paid for the surplus you export. This guide walks through the sizing logic, the numbers worth calculating before you ask for quotes, the Smart Export Guarantee, and the planning, grid and electrical rules, with the 2026 changes in England flagged where they exist.

How big a system?

Solar capacity is measured in kilowatts peak (kWp), the output of the array under standard test conditions. A typical modern panel is around 400 to 450 W, so a 10-panel array is about 4 to 4.5 kWp, and many UK semis fit somewhere between 3 kWp and 6 kWp depending on roof size. Usable area matters: a panel is roughly 1.7 m by 1.1 m, so ten panels need around 19 m² of unshaded roof.

The right size depends on two things: how much roof you have, and how much electricity you use. A home that is empty during the day and uses little power will export most of what it generates. A large system on such a house still helps, but its payback is slower, because exported electricity is paid at a lower rate than imported electricity costs. Adding a battery shifts more generation into the evening.

Estimating generation

In the UK, a well-placed south-facing array at a pitch of around 30 to 40 degrees with no shading typically produces somewhere in the range of 850 to 1,000 kWh per kWp per year in the south of England, with lower figures in the north and in Scotland. East-west roofs produce a little less in total but spread output across the day, which can suit a household that is home in the mornings and evenings. Shade from chimneys, trees or dormers can cut output a lot, which is why micro-inverters or optimisers are sometimes used.

Example with assumed numbers: a 4 kWp array at 900 kWh per kWp per year yields 4 × 900 = 3,600 kWh a year. If the household uses 40% of it directly (1,440 kWh) and exports the rest (2,160 kWh), the value depends on prices. At an assumed import price of 24p per kWh, the self-used part saves 1,440 × 0.24 = £345.60. At an assumed export rate of 12p per kWh, the exported part earns 2,160 × 0.12 = £259.20. Total is about £605 a year. The solar PV annual generation calculator lets you vary orientation, tilt and size, and then you can compare the saving with your quotes.

Your self-consumption figure is the big unknown. Working from home, running a heat pump or charging an electric car in the day lifts it; being out all day lowers it. Timer-controlled appliances, an immersion diverter that sends surplus to the hot water cylinder, and a battery all raise it.

Costs and payback

Installed prices vary by region, roof complexity, scaffolding, panel quality and whether a battery is added. Do not rely on a single average figure; get three quotes and check what is included: scaffolding, inverter, mounting system, DNO paperwork, MCS certificate, warranty and monitoring. Simple payback is the installed cost divided by annual benefit. Using the example benefit of £605 above, a system priced at £6,000 would give £6,000 ÷ £605 ≈ 9.9 years. These are illustrative numbers only. Panels typically carry product warranties of ten to twenty-five years and performance warranties of 25 years or more, and inverters often need replacing once during the panels' life.

In Great Britain the installation of solar panels in homes is currently zero-rated for VAT under a temporary relief that runs to 31 March 2027, after which the 5% reduced rate is due to return. The relief applies to installations by VAT-registered businesses. Policy can change, so confirm on GOV.UK when you are close to signing.

The Smart Export Guarantee

The Smart Export Guarantee (SEG) is how small generators are paid for electricity exported to the grid. It replaced the feed-in tariff for new installations. Ofgem's guidance explains that licensed electricity suppliers above a certain size must offer an export tariff, and that any SEG tariff offered must be above zero. Beyond that, suppliers set their own rates and terms, which differ widely. Some offer a flat rate, some offer time-of-use or smart tariffs, and some require you to be their import customer while others do not. Eligible technologies include solar PV up to 5 MW capacity, with the installation located in Great Britain, and a registered generator is normally required to have an MCS-accredited installation or equivalent.

Compare tariffs yourself rather than defaulting to your current supplier. Ask:

  • Is the rate fixed, or does it vary with time of day?
  • Is there a requirement to take their import tariff?
  • Do they require a smart meter capable of half-hourly export readings?
  • How and how often are payments made?

Rates move, so check current offers before you sign a contract and revisit them yearly.

Planning permission

Roof-mounted solar panels on houses in England are generally permitted development, subject to conditions such as not projecting too far from the roof plane and sitting below the highest point of the roof. Listed buildings and certain designated land have extra restrictions. The government published changes to the permitted development rules for domestic solar in mid-2026, taking effect on 27 August 2026 according to the Planning Portal. They separate the rules for houses and flats, clarify plug-in solar, and include a transitional period for projects which would otherwise be affected. I could not verify the exact dimensional limits (such as how far panels may project) in the amended order, so check the Planning Portal or your local planning authority for the current conditions before you start. Scotland, Wales and Northern Ireland have separate planning rules.

Grid connection and electrical safety

  1. Tell the Distribution Network Operator (DNO). Small installations up to a set limit per phase usually fall under the Engineering Recommendation G98 and are notified to the DNO within 28 days of commissioning. Larger systems need approval before connection under G99. Your installer normally handles this.
  2. Electrical installation to BS 7671. The inverter, AC isolators, protective devices and cabling must meet the wiring regulations. The work is notifiable under Part P in England (see our Part P guide), usually handled through the installer's competent person scheme membership.
  3. Structure. Panels and mounting add load to the roof. A competent installer checks that rafters and fixings cope with wind uplift and dead load. Old or weak roofs may need strengthening first.
  4. Fire safety. DC cabling needs proper routing and isolation, and the guidance on DC isolators and signage should be followed. The Planning Portal notes plug-in solar is not permitted on wooden structures or cladding for fire reasons.
  5. Building regulations. Installing panels on a roof can need building regulations approval in some cases, though a competent person scheme can self-certify. Ask your installer which route they use.

MCS and consumer protection

For SEG payments and many grant schemes you need an installation approved under the Microgeneration Certification Scheme (MCS). Check the installer's MCS certification and membership of a consumer code such as the Renewable Energy Consumer Code, and make sure the contract spells out the deposit, warranties and what happens if the installer ceases trading. Be wary of sales pressure, claims of “free solar” and lease deals that tie the roof to a long contract; read the terms about selling the house, since a lease can complicate a sale.

Solar plus other upgrades

Solar works well alongside a heat pump (see our heat pump comparison) or an electric vehicle charger, since both increase daytime and evening demand. A home charger can be set to charge when the sun is shining, or you can use a diverter that adjusts charging to the surplus. Read the EV charger installation guide for the circuit and smart charging rules. Improving insulation first, such as loft insulation, reduces your baseline demand and is often the cheaper first step.

Batteries: worth it or not?

A home battery stores surplus daytime generation for use in the evening, raising the share of solar you use yourself. The trade-off is cost: batteries add a substantial amount to the installed price and have their own warranty and cycle limits, so the extra saving rarely pays for itself quickly on solar alone. They make more sense where export rates are low, where a time-of-use import tariff lets you also charge from cheap overnight power, or where you value back-up during outages (which needs a specific configuration). Ask each installer to model your system with and without a battery using your own half-hourly usage if you have a smart meter, and compare the difference in annual benefit against the extra price.

Before you sign: a short checklist

  • Get at least three MCS quotes with a stated annual generation estimate and the assumptions behind it (orientation, tilt, shading).
  • Ask for the proposed inverter and panel makes and read the warranty terms.
  • Check your roof's condition and any scaffolding cost.
  • Find out how the installer handles DNO notification and Part P.
  • Compare SEG tariffs.
  • Check your buildings insurance and mortgage terms.
  • Understand what happens in a power cut: most grid-tied systems shut down for safety unless specially configured.

This is general information, not financial or electrical advice. Savings depend on your use, your tariffs and your roof, and prices change.

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