Adding More Solar Panels to an Existing System

How to add more solar panels to an existing system

Need more power and wondering if you can just add some more solar panels to your existing system? For most systems it’s fine, and the norm. The things to check first are your inverter (they’re limited), your grid connection, and whether you have the older, more valuable Feed-in Tariff.

Here’s everything to check, the costs, and some viable routes.

What really decides it

Four things settle whether an expansion is straightforward, awkward, or best done a different way. In rough order of how often they cause problems: your inverter’s spare capacity, the grid connection rules, your Feed-in Tariff status, and how well new panels match the old ones. Roof space matters too, but if you’re reading this you probably have some.

Panel matching, and why it’s less strict than it sounds

The old advice was to use identical panels. That’s ideal but rarely possible, because the exact model you fitted years ago is usually discontinued. Panels wired in the same string tend to perform at the level of the weakest one, so a mismatch there can drag output down. A competent installer designs around this: new panels go on their own string or tracker, or on microinverters that let each panel work independently. Do that, match the voltage and current characteristics as closely as you can, and a slight difference in wattage or age costs you very little.

Roof space and the boring structural check

A rough guide is that each extra kilowatt of panels needs around 5 to 6 square metres of unshaded roof. If your first array only covered the south pitch, an east or west face is usually worth using: it produces around 20% less than south, but it spreads generation across the morning or evening when you’re more likely to use it. A quick structural check confirms the roof can carry the extra load, which is minor for a handful of panels but not something to skip. If your roof is completely full, a ground mount or a solar carport can carry the extra capacity instead.

Will your inverter take more?

This is the question that most often decides the shape of the job. Your inverter converts the DC electricity from the panels into AC your home can use, and every inverter has a ceiling. When your system was designed, the inverter was usually sized at roughly 80 to 120% of the panel array, which means many already run with some deliberate headroom.

Most string inverters will accept around 20 to 30% more panel capacity than their nameplate rating before you hit a hard limit, because panels rarely produce their full rated output in UK conditions. So a 3.68kW inverter can often carry around 4.4 to 4.8kWp of panels without complaint. Push past that and you either upgrade the inverter or add a second one. Your inverter’s label and datasheet give its maximum DC input and number of MPPT trackers; a spare tracker makes adding a separate new string far easier.

Grid permission: G98 vs G99

Every grid-connected system has to be registered with your Distribution Network Operator, the company that runs the cables in your area. The rules turn on how much your inverter can push onto the network. On a standard single-phase supply, up to 3.68kW of total inverter capacity can be connected under a G98 notification, which your installer can file after the work is done. Above 3.68kW you move into G99 territory, and that means applying for permission before the job goes ahead.

A G99 application is an approval, not a formality, and it can take several weeks or longer while the operator checks the local network can absorb the extra export. If your expansion tips you over the threshold, that wait belongs at the front of the project. Our explainers on the DNO approval process and on G98 versus G99 walk through the paperwork in detail.

The Feed-in Tariff trap

This is the one that catches people out, and it only applies if your system was installed and accredited before the scheme closed to new applicants in 2019. If that’s you, your Feed-in Tariff is likely paying you a generation rate that no new install can get anywhere near, sometimes ten times today’s export rates (the average FiT payment for a 10KW install from 2010 is £550 a month, and still going). Protecting that income is worth far more than the marginal generation from a few extra panels.

The rules themselves aren’t as scary as the rumours. Ofgem requires you to notify your FiT supplier of any change to your system’s total installed capacity, and you’re allowed to extend an accredited installation and keep the original accreditation. Your generation payments stay tied to your originally accredited capacity, at your original rate, for the rest of the term. What you can’t do is earn FiT on the new capacity.

The clean solution is to treat the new capacity as its own system. Wire the extra panels into a separate inverter, or into a hybrid battery system, so the original FiT meter carries on seeing only the original array. The new panels then earn their keep through the electricity you use directly and, if you give up FiT export payments, the Smart Export Guarantee, while your generation tariff stays untouched. It costs a bit more than bolting everything onto one inverter, and for a pre-2019 FiT home it’s almost always the right call. If your system is that vintage, our guide to upgrading an older system covers the wider picture.

What it costs in 2026

Here’s the uncomfortable truth about small expansions: the cost per panel can be higher than on a full install, sometimes much higher. Scaffolding, an electrician’s day, travel and paperwork barely change whether you fit four panels or fourteen. On a full install those fixed costs are spread across ten or more panels; on a top-up they land on a handful.

The example below assumes a straightforward two-storey house, an inverter with spare headroom, and no FiT complications.

Change the assumptions and the figure moves. A two-panel top-up on the same house would still cost around £2,000, because the scaffolding and labour have to be paid either way. An expansion that needs a new inverter typically runs £2,500 to £5,000, and a separate array paired with a battery, the FiT-safe route, more often lands between £4,000 and £8,000 depending on storage size. Residential solar work carries 0% VAT until 31 March 2027, after which it’s set to revert to 5%, so a planned expansion is cheaper done before that window closes. For the wider market context, see our full breakdown of what a solar installation costs.

Your four expansion routes

There are really four ways to add capacity. The three most common are below; the fourth, microinverters, suits shaded or awkward roofs where each panel benefits from working on its own.

The middle route wins for most homes because a modern hybrid inverter solves several problems at once: it carries the extra panels, it’s ready for a battery, and swapping an ageing inverter now avoids a separate replacement in a year or two. If a battery is on your mind either way, our guides to retrofitting a battery to existing solar and to AC versus DC coupled batteries explain how the wiring choices interact with an expansion.

More panels or a battery?

Before you commit to more panels, it’s worth asking whether they’re the best use of the money at all. If your existing system already exports a large share of what it generates, you’re effectively giving that electricity away at a low export rate. Adding panels makes that worse, not better, because you generate even more surplus you can’t use. In that situation a battery, which soaks up the surplus for evening use, often saves more per pound than extra generation.

More panels make the stronger case when your daytime demand has climbed, from an EV charging at home, a heat pump, or air conditioning, so the extra output gets used directly rather than exported. The honest answer for many homes is a bit of both: modest extra capacity plus storage. Check your own export figures first; if you’ve noticed your system producing less than expected, sort that out before adding to it, since a fault or heavy shading will drag new panels down too.

How to do it properly

Work through these steps in order and you avoid the two classic failures: a stalled grid application and a damaged tariff.

Frequently asked questions