Extending an existing solar PV system

The first is to add panels to the existing inverter. That only works if the inverter has spare input capacity and a free input for a new string, and if the new panels are electrically compatible with the setting of that input. Panels made a decade ago differ from current ones in size, voltage and current, so new panels are rarely added to an old string.

The second is a separate system alongside the old one: new panels, usually on another roof slope, a garage or a garden frame, with their own inverter or microinverters. The two systems then run independently and are metered according to what your contracts require.

The third is replacement. If the original inverter is near the end of its life and the panels are small and dated, taking everything down and designing one new system can be simpler and, over the life of the system, not much dearer than an extension. A fair installer will price both options when the old equipment is marginal.

If your original system earns Feed-in Tariff payments, be careful. The scheme closed to new applicants on 1 April 2019, and extensions to accredited installations commissioned on or after 15 January 2016 are not eligible for FIT payments. You must tell your FIT licensee when you add generating equipment, and the metering has to be arranged so that the FIT payments stay limited to the original installation. Ask the installer and the licensee how that will be done before you sign; mistakes here can put your existing payments at risk.

Under the Smart Export Guarantee, capacity can be added to an eligible installation, and your SEG contract requires you to tell the supplier about extensions as soon as reasonably possible. The new panels need their own MCS certificate if you want the export from them to count.

You cannot take FIT export payments and SEG payments for the same installation. An installer who works with extensions regularly will know how to handle this combination.

Source: gov.uk · ofgem.gov.uk

What the network operator looks at is the total capacity at your address, not just the new panels. An extension can move you from the simpler connect-and-notify route into one where the installer has to apply and wait for approval before installing. If capacity on your local network is tight, the operator may ask for an export limit. The quote should state the combined capacity and the route that applies.

Additional panels on another slope are subject to the same permitted development conditions in England as the original ones, and the same restrictions in conservation areas and on listed buildings. A garden array at a house has its own limits, including a maximum panel area of 9 square metres. The roof structure has to carry the extra load, so the survey should cover it again rather than rely on the original installer's assessment.

Scaffolding is often a large part of the price of a small extension. Combining the job with other roof work, or with a battery fitted at the same visit, can make it better value.

• the original MCS certificate, inverter model and number and type of panels
• Feed-in Tariff or Smart Export Guarantee contract details
• your recent electricity use and any new loads such as an electric car
• photos of the inverter label, the consumer unit and the meter
• the roof slopes or outbuildings you are considering
• whether a battery is planned now or later

Your description goes, without anything that identifies you, to solar installers in your area once you have confirmed your number with a text code and your email with a link. The first installer to buy it is the only one; the request is then taken off the shop, and the installer pays for it rather than you.

We send you the installer's name, address and phone number by email before it sees your details. If no installer buys within 21 days, the request is withdrawn, and there is never an obligation to accept a quote. We check that buyers are registered businesses, not their workmanship, so confirm the MCS certification on the MCS website.