Solar panels

Solar cells produce direct-current electricity. An inverter converts it into alternating current that household appliances can use. Electricity is used in the home first; any shortfall comes from the grid and any surplus can be exported or stored in a battery.

System size is normally shown in kilowatts peak, written as kWp. This is a rated output under test conditions, not the amount the system will produce continuously. Generation is measured in kilowatt-hours, or kWh, just like electricity use on a bill.

A credible quotation should provide an annual generation estimate for the actual roof. It should account for orientation, pitch, shade and system losses. It should not multiply the panel rating by every hour of the year.

Roof direction and pitch

South-facing roofs often produce the most electricity over a year, but south-east, south-west, east and west can all work. East-facing panels generate more in the morning; west-facing panels generate more later in the day. A split east-west system may align better with household demand even if its maximum output is lower.

Shade

Trees, chimneys, dormers and nearby buildings can reduce output. Shade that moves across a small part of an array can affect more than one panel, depending on the system design. Ask the installer to show how shade has been modelled and whether optimisers or microinverters are justified.

Roof condition and usable area

Panels commonly remain in service for decades, so roof repairs are best considered before installation. The survey should check the roof covering, structure, access, panel layout and space required around obstructions. Flat roofs can also be suitable, but mounting, wind loading and spacing need careful design.

Planning and permissions

Many domestic installations fall within permitted-development rules, but conditions vary across the UK and may be different for listed buildings, conservation areas, flats and unusual mounting positions. Leaseholders and tenants may also need the freeholder’s or landlord’s consent. Check the relevant planning authority before committing.

Is your roof suitable for solar? The full checklist →

The price of a solar installation depends on system size, panel and inverter choice, roof access, scaffolding, electrical work, monitoring, bird protection and whether a battery is installed at the same time.

Compare at least three written quotations on the same basis. Each quotation should state:

• the number, model and rated power of the panels;
• total system size in kWp;
• inverter type and rated capacity;
• predicted annual generation and the assumptions behind it;
• the effect of shade;
• scaffolding and access costs;
• consumer-unit or other electrical work;
• monitoring equipment and app access;
• product and workmanship warranties;
• who deals with the distribution network operator;
• expected completion, commissioning and handover documents; and
• the full price, including VAT and any optional extras.

Do not compare quotations only by price per panel. A smaller number of higher-powered panels may produce more than a larger number of lower-powered panels, and a cheaper quotation may exclude scaffolding, electrical upgrades or aftercare.

Estimate your solar generation and its value →

The electricity generated has two possible values.

Electricity used immediately in the home avoids buying that amount at the import rate. Electricity exported earns the applicable export rate. The same unit cannot be counted as both self-used and exported.

Annual benefit = electricity self-used × import rate + electricity exported × export rate

For a more realistic estimate:

1. Use an installer’s generation estimate for the roof.
2. Estimate how much generation will be used while it is produced.
3. Apply the household’s actual import rate to self-used electricity.
4. Apply a realistic Smart Export Guarantee rate to exported electricity.
5. Allow for inverter replacement, maintenance and any finance cost.
6. Test more than one electricity-price and usage scenario.

Payback is not guaranteed. It changes when tariffs, household routines, system performance or repair costs change.

A battery stores surplus electricity for use later. This may reduce evening imports and can work with a time-of-use tariff, but the financial case is separate from the case for panels.

Check:

• usable rather than advertised battery capacity;
• maximum charge and discharge power;
• round-trip efficiency;
• expected cycles and warranty terms;
• whether the system can charge from the grid;
• whether it can operate during a power cut;
• compatibility with the inverter, tariff and smart meter; and
• the cost of replacing the battery or inverter later.

Backup power is not automatic. Many ordinary solar and battery systems shut down during a power cut to protect engineers working on the network. If backup matters, it must be designed and quoted explicitly.

Check battery savings for your home →

No solar panels? Check a battery charged at a cheaper rate →

The Smart Export Guarantee requires eligible electricity suppliers in Great Britain to offer a payment for eligible small-scale renewable electricity exported to the grid. Rates and conditions are set by suppliers, not by Ofgem, and can vary substantially.

Northern Ireland has a separate electricity market and the Smart Export Guarantee does not apply there. Check current export arrangements with the relevant supplier and the Utility Regulator.

An export arrangement may require an eligible installation, an export-capable smart meter and evidence such as an MCS certificate. The export supplier does not always have to be the same company that supplies the home’s electricity, although some tariffs impose linked-product conditions.

Compare the export rate alongside the import tariff. A high export rate is not automatically the best overall deal if the linked import tariff is expensive.

Compare solar export tariffs →

The installer should survey the property, design the system, explain expected performance, agree the cable route and organise any required network notification or permission. After commissioning, keep the electrical certificates, warranties, equipment details, performance estimate, MCS certificate where applicable and instructions for monitoring and shutdown.

Use an MCS-certified installer when certification is required for a grant or export tariff. Check the company and the individual quotation rather than relying on a logo in an advert.

Solar installation and handover checklist →

Solar panels have no moving parts, but the system is not maintenance-free. Monitor generation so a fault is noticed, keep nearby trees under review and follow the manufacturer’s inspection guidance. Cleaning is not automatically required; its value depends on dirt, roof angle, access and local conditions.

Panels usually carry long product and performance warranties. Inverters often have shorter warranties and may need replacement during the life of the panels. Read what each warranty covers, who provides it and whether labour or access costs are excluded.

Solar panels not generating enough? →

• Is the roof structurally sound and likely to remain untouched for the life of the system?
• Has shade been measured rather than guessed?
• Is predicted generation based on the actual roof?
• How much electricity is used during daylight hours?
• What import and export rates have been assumed?
• Is a battery being judged separately from the panels?
• Are scaffolding, electrical work and monitoring included?
• Is the installer appropriately certified and insured?
• What happens if the installer stops trading?
• Which equipment may need replacing, and when?