Since plug-in solar became legal in Great Britain in August 2026, a question has started appearing that did not exist before: should I buy a £600 kit or a £7,000 installation? Put like that it sounds as though these are two versions of the same purchase. They are not. The useful comparison is not which is better, because a rooftop system is better on almost every measure, but which one fits the situation you are actually in.

The Difference in Scale

The single most important number is size. A plug-in kit is capped by law at 800W of inverter output. A common rooftop installation on a UK semi is around 4kW, and larger roofs take 5kW or 6kW. An 800W kit is therefore about a fifth of a standard system, and everything else follows from that.

  • Estimated annual generation: roughly 600 to 700kWh for an 800W kit in good conditions, against roughly 3,400kWh for a 4kW roof array.
  • Typical installed cost: roughly £450 to £650 for a kit, against roughly £5,500 to £8,000 for a 4kW system.
  • Estimated annual saving: around £43 to £84 for a kit, against several hundred pounds for a roof array.
  • Indicative payback: roughly seven to fourteen years for a kit, roughly seven to ten years for a well sited roof system.
  • Export income: none for a plug-in kit, available through the Smart Export Guarantee for an MCS certified rooftop installation.
  • Portability: a kit can be removed and taken to a new home, a roof system cannot.

Note what those figures do to the usual intuition about cheap versus expensive. The kit costs a tenth as much but does not have a payback period a tenth as long. It is roughly comparable, and in poor conditions worse, because so much of the cost of a small kit is fixed and because none of its surplus earns anything.

Export Payments Change the Arithmetic

A rooftop system fitted by an MCS certified installer qualifies for the Smart Export Guarantee, under which suppliers pay you for units you send to the grid. Rates vary considerably by supplier and tariff. A plug-in kit does not qualify, because the SEG requires certified installation and a self installed kit by definition does not have it.

The practical consequence is that a plug-in kit only pays you for electricity you consume at the moment it is generated. Everything else is given to the grid for nothing. This is why plug-in savings are so sensitive to whether anyone is home during the day, and why the sensible way to size expectations is to think about your constant background loads rather than your total consumption. Our guide to what MCS certification means explains why that requirement exists.

When a Plug-In Kit Is the Right Answer

There is a substantial group of people for whom the rooftop option does not exist, and for them the comparison is with doing nothing rather than with a 4kW array. A kit is the sensible choice if you rent, if you own a leasehold flat with no rights over the roof, if your roof faces north or is heavily shaded, if your roof needs replacing before anything could be fitted to it, or if a five figure outlay is simply not available to you at the moment. Our guide to balcony solar for renters and flat owners covers that situation in detail.

A kit also functions reasonably as a trial. If you are unsure whether solar suits your household, £600 and an afternoon is a cheaper way to find out how much daytime generation you actually use than committing to a full installation. The kit remains useful afterwards, or can be sold on.

When the Roof Is the Better Investment

If you own a house with an unshaded roof facing anywhere from east through south to west, the rooftop option is materially better and the plug-in kit is a distraction. You get roughly five times the generation, you get paid for the surplus, the system is warranted and installed by a certified firm, and the payback period is comparable or shorter despite the much larger outlay. Our 2026 cost guide sets out what a fair quote looks like.

There is also a timing consideration. The 0% VAT rate on residential solar is currently scheduled to end on 31 March 2027, after which it is due to return to 5%. On a £7,000 installation that is roughly £350. If a rooftop system is the right answer for your home, that date is worth having in mind. The wider policy picture is covered in our summary of the 2026 solar rules.

Be wary of anyone presenting a plug-in kit as equivalent to a rooftop system, or quoting its savings without mentioning that none of the surplus is paid for. The two products serve different situations and an honest seller will tell you which one you are in. The same applies in reverse: an installer who dismisses plug-in solar entirely may simply not sell it.

Can You Do Both?

It is not a sensible combination. The grid rules limit you to one plug-in kit per household, and if you already have a rooftop array there is little point adding 800W of unmetered generation that earns nothing, when extending the certified system would earn export income on everything above your own use. Where people do sometimes run both is during a transition, keeping a kit going while waiting for a rooftop installation to be scheduled, and then selling it on.

Start by Finding Out Which Group You Are In

Almost all of this resolves to one question: is your roof usable? If it faces a reasonable direction and is not badly shaded, look at a proper installation and treat plug-in solar as a stopgap at most. If it does not, or if the roof is not yours to use, an 800W kit is now a real option where previously there was none. Check your roof orientation with our free tool, which takes a few seconds and costs nothing, before you spend money on either.

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Is a plug-in kit a good way to try solar before committing?

It is a reasonable one. For around £600 you learn how much of your daytime generation your household actually consumes, which is the variable that drives the return on a much larger system. The limitation is that an 800W kit tells you little about how a 4kW array would behave in summer, when a full system frequently generates more than a home can use and the export rate starts to matter.

If I install a plug-in kit now, can I add a rooftop system later?

Yes, and the kit does not interfere with a later installation. When the rooftop system is commissioned it is notified separately by your installer, and you would normally remove the plug-in kit at that point, since keeping it adds generation that earns nothing while your certified system would be paid for the same surplus. The kit can be sold on or taken to another property.

Why is the payback period on a cheap kit not much shorter?

Because the savings scale down with the system size but some of the disadvantages do not. A plug-in kit generates around a fifth of what a 4kW array does, and unlike the larger system it earns nothing at all for surplus generation. A £600 outlay saving an estimated £43 to £84 a year gives roughly the same payback period as a £7,000 outlay saving several hundred, which is why price alone is a poor guide to value here.