Rooftop solar explained

How big a system?

The size depends on your roof space, which way it faces, your budget and how much electricity you use. The Energy Saving Trust gives about 4.5 kWp, around 12 panels, as typical, costing around £7,600. As a rough guide, a well-sited UK system makes around 750 to 1,100 kWh a year for each kWp. Ofgem’s figure for a typical home’s use is 2,500 kWh a year, but you won’t use all your solar electricity as it is made, so a bigger system mainly means more exported unless you add a battery or use more in the daytime.

Choosing an inverter

• String inverter: panels are wired together into one central inverter. Usually the simplest and cheapest, but shade on one panel can pull down the output of the others on that string.
• Microinverters: a small inverter behind each panel. Shade or a fault affects only that panel, and each panel can be monitored, but there is more equipment on the roof.
• Power optimisers: a small box behind each panel, still feeding a central inverter. A middle way for partly shaded roofs.
• Hybrid inverters also connect a battery, now or later. See Home batteries.

The Energy Saving Trust says an inverter usually needs replacing after around 12 years, so check its warranty and budget for it.

Inverter size and the grid

On a normal single-phase supply, a system with an inverter of up to about 3.68 kW can be connected first and then notified to your network operator under G98. A bigger inverter needs the operator’s approval under G99 before it is installed. That is why many systems of around 4 to 5 kWp of panels use an inverter of about 3.6 kW: the panels rarely produce their full rating, so little is lost.

Planning and building rules

In England, rooftop panels on a house are usually permitted development if, on a pitched roof, they are no higher than the highest part of the roof and stick out no more than 200 mm, and, on a flat roof, they are no more than 600 mm above the highest part. Listed buildings, conservation areas and Article 4 areas are different. Wales, Scotland and Northern Ireland have their own rules. The electrical work must meet building regulations, and the installer should check that your roof can take the extra weight. See Do I need permission?

Paperwork you should be given

The Renewable Energy Consumer Code, which many MCS installers follow, requires members to hand over:

• full operating and maintenance instructions;
• signed commissioning records;
• the MCS certificate and a compliance certificate;
• the invoice.

These must not be held back until you have paid in full. Also keep confirmation that your network operator has been notified, the warranties for the panels, inverter and any battery, and the workmanship guarantee. See Choosing a good installer.

Being paid for exports

Under the Smart Export Guarantee, suppliers in Great Britain with at least 150,000 domestic customers must offer a tariff for the electricity you export, and the installation needs an MCS (or equivalent) certificate. Rates vary, so compare them. Northern Ireland is different. See Tariffs and export payments.

Insurance and your mortgage

Tell your home insurer before the panels go up, and check your mortgage terms, as lenders often expect to be told. Be very careful with “free panels” offered in return for leasing your roof: it is a long-term, legally binding lease, and lenders expect to be asked for consent. Take independent legal advice before signing one.