Adding plug-in solar to rooftop solar

The short answer, and why it is not shorter

There is no single published rule we can point you to. There are two statements, from bodies that both have standing, and they pull in different directions.

The Energy Networks Association — the body for the companies that run the distribution networks, and the operator of the device registration service — answers the question like this:

“If you already have a 3.68 kW G98 solar system at your property, you must use a qualified installer to apply for G99 approval. Any additional generation must be wired in and not a plug-in installation.”

Energy Saving Trust, in guidance written with the Department for Energy Security and Net Zero, answers it like this:

“This is usually possible. You should speak to the original installer or a qualified electrician to check that your new plug-in solar panels won’t overload your system. You may need to notify your Distribution Network Operator (DNO) if adding the plug-in solar panel device means your home is exporting electricity over a certain amount.”

Both were published or updated within days of each other in early October 2026. We are not going to reconcile them for you, and we would be suspicious of anyone who does.

What the difference actually turns on

Read the two quotes again and notice that the ENA’s is conditional. It describes a home that already has a 3.68 kW G98 system — in other words, a home already at the top of what the smaller connection route allows. Energy Saving Trust’s answer is general and adds a check rather than a threshold.

The thing that makes the ENA’s answer possible at all is that the G98 route is about your installation, not about each device. Its full title, as quoted in Ofgem’s decision on the plug-in solar changes, is “requirements for the connection of Fully Type Tested Micro-generators (up to and including 16 A per phase) in parallel with public Low Voltage Distribution Networks”. Sixteen amps per phase is roughly 3.68 kW. If the roof has already used that up, a plug-in kit is not a separate small thing that sits outside it — it adds to the same total.

We could not read the G98 document itself: the ENA publishes the engineering recommendations through the Distribution Code rather than as a public web page, and its site does not serve them to us. So we are not going to state the combined-capacity rule as though we had read it. What we can say is that the ENA, on its own registration service, applies it that way.

What to do, depending on your system

Find the rated output of your existing system — it is on your installation paperwork, your MCS certificate or your inverter, and your installer will know it.

• At or near 3.68 kW. Take the ENA’s answer seriously: ask your distribution network operator before you spend anything. If the answer is that additional generation needs a G99 application, a plug-in kit is not the route, and the honest comparison is against adding a string to the existing system instead.
• Comfortably below it — a 2 kW array, say, or a few panels added years ago. Energy Saving Trust’s route applies: ask the original installer or a qualified electrician whether the addition will overload anything, and register the device as normal. Our registration guide covers that part.
• You do not know. Do not guess from the number of panels: a panel count tells you very little about rated output. Ask, then decide.

Your network operator is the company that owns the cables in your street, not the supplier you pay. Our G98 guide explains how to identify them.

Four things that catch people out

• An export limit on the existing system. Plenty of rooftop installations were approved with their export capped in the inverter settings. Adding generation behind that cap can mean the new kit’s output is curtailed rather than used, which destroys the payback sum. See export limitation.
• A G99 connection changes the question entirely. If your existing system went in under G99 — larger systems, or a battery — the terms of that approval govern what else may connect. Go back to them, or to your DNO, before anything else.
• Your export arrangement. If you are paid for export under the Smart Export Guarantee or a legacy Feed-in Tariff, adding uncertified generation behind the same meter may affect it. We are not going to guess how: ask your supplier, in writing, and say only what they tell you. See the Smart Export Guarantee.
• The kit still has to be a compliant kit. Nothing about already owning solar relaxes the rules for the new device: one plug-in device per household, 800 VA maximum output, no battery storage, and the exact model listed as compliant on the ENA register.

When another string beats a plug-in kit

If you already have rooftop solar and an installer you trust, a plug-in kit is not automatically the cheapest way to add generation. Worth pricing both:

• Adding panels to the existing system keeps everything under one approval and one export arrangement, and the marginal cost per watt can be lower than a kit — if the inverter has headroom and a spare string. Ask your installer what the inverter will take.
• A plug-in kit wins when the roof is full, badly oriented or shaded, when the good sun is at ground level, or when you want something you can take with you. It is also the only route that does not need an installer at all.

For the broader comparison of the two routes, see plug-in solar versus rooftop solar.