Can You Add More Solar Panels to an Existing System?

Usually, yes. Most solar systems can be extended, and plenty of people add panels years after the first ones went up, often when they get an electric car, a heat pump or a battery.

Four things decide how it can be done: whether there is space on the roof, what your existing inverter can take, what your grid connection allows, and, if your panels are on the Feed-in Tariff, how the new panels affect your payments. None of them is usually a dead end. Each one shapes which option makes sense.

Key points

• Your existing panel model may no longer be made, so new panels usually go on their own string or their own inverter.

• Your inverter has a limit on how much it can take. Adding panels can mean using a spare input, adding a second inverter or replacing the inverter.

• The grid limit applies to all the generation at your property, old and new together. Going over it means a network operator application before the work.

• On the Feed-in Tariff, panels added since 15 January 2016 earn no FIT payments. Your existing FIT carries on.

• If adding panels is mainly about using more of your own electricity, a battery may do more for you.

Is there room on the roof?

Start with the simple question. New panels need unshaded roof space facing a useful direction, clear of the edges, chimneys and vents. East and west facing slopes can work well alongside an existing south-facing array, because they generate more in the mornings and evenings.

It is also worth checking the roof itself before adding anything. If the roof will need work in the next few years, doing it before or at the same time as the new panels saves taking everything down twice. Our guides to whether you need a new roof before solar and removing panels for roof repairs explain the trade-offs.

Will the new panels match the old ones?

Probably not exactly. Panel models change quickly, and the one on your roof may have been discontinued years ago.

That matters because panels wired together in a string should be electrically matched. Mixing a new, higher-powered panel into an older string can hold the string back to the weakest panel’s level. So new panels normally go on their own string, either on a spare input on your existing inverter or on a new inverter. Done that way, mismatched panels are not a problem.

What can your inverter take?

Every inverter has limits: how much panel power it can accept, and how many separate inputs (MPPTs) it has for strings. That gives you three broad options.

1. Use spare capacity on your existing inverter. If the inverter has an unused input and headroom, the new string can connect to it. This is the simplest route, but older systems were often sized closely, so there may be no spare capacity. Some deliberate oversizing is normal and safe within the manufacturer’s limits; our guide to solar panel oversizing explains how it works.

2. Add a second inverter for the new panels. The new array gets its own inverter and the original system is left alone. This keeps the two systems separate, which can help on the Feed-in Tariff (see below).

3. Replace the inverter with a larger or hybrid one. If your inverter is getting old, replacing it with one that can take both arrays, and a battery, can make more sense than working around it. The Energy Saving Trust puts a typical inverter life at around 10 to 15 years, shorter than the panels. If you are on the FIT, read our guide to replacing an inverter on a Feed-in Tariff system first.

Your grid connection limit

Before any solar system is connected, the local network operator (UK Power Networks for most of London) either has to be told about it or has to approve it. The limit that decides which applies is for all the generation at your property, not just the new panels.

So a system that sat comfortably within the simpler notification route on its own can be pushed over the limit by an extension. When that happens, an application has to go to the network operator and be approved before the new panels are connected. Sometimes the answer is an export limit, which lets you install more panels than the connection would otherwise allow.

Our guide to solar export limits, G98, G99 and DNO approval explains how the limits work.

If you are on the Feed-in Tariff

This is the one to check most carefully, and it is worth talking to your FIT licensee before the work.

• Your existing FIT carries on. Adding panels does not end it.

• The new panels earn no FIT. Ofgem’s guidance is that an extension to a FIT installation commissioned on or after 15 January 2016 is not eligible for FIT payments.

• Shared meters are split. If the new panels share a generation meter with the old ones, the readings are divided in proportion to the installed capacity of each, and FIT is paid only on the original share.

The new panels still pay their way through lower bills and, with the right set-up, export payments. Our guide to keeping your Feed-in Tariff when you upgrade covers the metering and export options in detail.

If you are not on the Feed-in Tariff

Things are simpler. To be paid for exported electricity under the Smart Export Guarantee, your installation and installer need to be MCS certified, so make sure the extension is installed and certified by an MCS-certified installer. Then tell your SEG supplier about the change.

Do you need planning permission?

Most London homes can add panels under the same permitted development rules that covered the first ones, but conservation areas, Article 4 directions and listed buildings change the picture. Our guides to planning permission for solar in London and Article 4 directions explain what to check.

Would a battery do more for you?

It depends on why you want more panels. If you already export a lot of electricity in the middle of the day and buy it back in the evening, more panels will mostly mean more export. A battery stores that surplus for the evening instead. If your roof is full, your connection is limited or your use has grown with an EV or a heat pump, the answer may be both, or a battery first.

Our guide to adding a battery to existing solar panels explains the options.

Frequently asked questions

Can I mix different panels on the same roof?

Yes, as long as they are wired correctly. New panels normally go on their own string or inverter so that they don’t hold back the old ones.

Will adding panels stop my Feed-in Tariff?

No. Your existing FIT continues. The new panels just don’t earn FIT themselves.

Do I need a new MCS certificate?

The extension should be installed and certified by an MCS-certified installer, which is what you need for export payments on the new panels.

Can you extend a system you didn’t install?

Yes. We survey the existing system first, then design the extension around it.

Request a solar and battery quotation. We’ll look at your roof, your inverter and your connection, and tell you which way of extending makes most sense.