The arrival of plug-in solar in the UK does not make rooftop solar irrelevant. They are different products aimed at different situations. Understanding what each one does well helps you work out which makes sense for your home, or whether there is a case for doing both.
Cost
A full rooftop solar installation typically costs between £5,000 and £10,000, including the panels, inverter, scaffolding and installation. A plug-in kit costs between £400 and £1,000 depending on the size and whether it includes battery storage.
The upfront difference is significant, but so is the output. A rooftop system on a typical UK semi-detached house might be 3.5 to 4kW, generating four to five times as much electricity as an 800W plug-in kit. The cost per kilowatt of installed capacity is broadly similar between the two approaches once you account for this difference.
Who can have each one
Rooftop solar requires ownership of the roof, planning permission in some cases, structural suitability, and a south-facing pitch. This rules it out for most renters, flat owners, and anyone in a property without a suitable roof.
Plug-in solar works for renters, flat dwellers, anyone with a balcony or garden, and homeowners who want to add generation capacity without the commitment and cost of a full installation. The key requirement is some south-facing outdoor space within cable distance of a socket.
Export payments
Rooftop solar systems that meet MCS (Microgeneration Certification Scheme) standards qualify for Smart Export Guarantee payments, meaning you are paid for the surplus electricity you send back to the grid. Rates vary by supplier but can add meaningfully to the annual return on a rooftop system.
Plug-in solar systems currently cannot achieve MCS certification, so SEG payments are not available. Any surplus you generate flows to the grid without payment. This makes self-consumption more important: the more of your own solar electricity you actually use, the better your return.
Installation and disruption
A rooftop installation typically takes one to two days, involves scaffolding, and requires electrical work inside your home including a new consumer unit connection. A plug-in system takes a couple of hours to set up and needs only a socket connection.
Which to choose
If you own your home, have a suitable south-facing roof and can afford the upfront cost, rooftop solar almost certainly gives you the better long-term financial return, particularly with SEG payments. If you rent, live in a flat, do not have a suitable roof, or simply want to start generating electricity quickly and cheaply, plug-in solar is the obvious starting point. The two are not mutually exclusive: some homeowners are adding a plug-in kit to a property that already has rooftop panels, using it to add capacity on a south-facing wall or outbuilding.