Last reviewed: 4 September 2026. Figures and legal points checked against primary sources (legislation.gov.uk, gov.uk, DESNZ, the Energy Networks Association and Ofgem). Where a number comes from our own calculation rather than a government document, we say so.
These are not two sizes of the same product. A plug-in kit is an appliance: 800 VA of AC output, up to 2,000 W of panel, no certification, no export payment, and you can take it with you when you move. A rooftop array is a building alteration: five times the capacity or more, MCS certification, an export meter point, income under the Smart Export Guarantee, and it stays with the house. If you own the roof and intend to stay, rooftop wins on almost every measure that matters. If you rent, live in a flat, cannot get scaffolding onto the roof, or simply will not spend four figures on a house you might sell, plug-in is the only one of the two you can actually have.
The short answer, by household
- Owner-occupier, unshaded roof, staying five years or more. Rooftop. The certification unlocks export income that a plug-in kit can never earn, and the per-kWh cost of the system falls as it gets bigger.
- Renter, or leaseholder in a flat. Plug-in, if you can get consent for the mounting. Rooftop is not available to you on any realistic terms.
- Owner-occupier who is moving within two or three years. Plug-in. It is portable; a rooftop array is not, and the evidence that it lifts a sale price is not something we are willing to assert.
- Anyone whose roof faces north, is heavily shaded, or is structurally unsuitable. Neither on the roof — but a ground-mounted plug-in frame in a sunny part of the garden may still work. See ground-mounted plug-in solar.
The comparison, line by line
| Criterion | Plug-in solar | Rooftop solar | Where this comes from |
|---|---|---|---|
| AC output ceiling | 800 W (expressed as 800 VA apparent power), maximum 3.5 A into the socket | No statutory ceiling; typically 3–5 kW inverter on a domestic roof | SI 2026/848 definition; Interim Product Specification clause 4.1 |
| Panel capacity | Up to 2,000 W DC, maximum four modules, no more than two in series per string | Limited by roof area and DNO agreement, not by product rules | Interim Product Specification clauses 4.1 and 4.2.2 — not in the SI |
| Who installs it | You. Designed for “installation and operation by ordinary persons” | An MCS-certified installer, with scaffolding and fixed wiring work | Interim Product Specification, scope |
| Network paperwork | G98 notification to your DNO within 28 days, after the fact, free | G98 notification too, but done by the installer as part of the job | ENA Engineering Recommendation G98 Issue 2 Amendment 1 |
| How many you may have | One per household under current G98, whatever the specification permits | One system, sized to the roof | Interim Product Specification, scope note on G98 |
| Battery | Not permitted in the device, and the instructions must warn against pairing one | Permitted, common, and eligible for zero-rated VAT until 31 March 2027 | SI 2026/848 definition; spec clause 8.3.2; VAT Notice 708/6 |
| Export income | None. No MCS certificate, no export MPAN, no SEG tariff | SEG payments from a licensed supplier for metered export | Ofgem, Smart Export Guarantee eligibility |
| Where the saving comes from | Self-consumption only — every exported unit is worth £0 | Self-consumption plus export, so surplus still has value | Our reading of the above |
| Reversibility | Attachment must be “reversible and non-permanent”; unplug it and take it | Permanent. It belongs to the building | Interim Product Specification clause 8.3.2 |
| Buildings it may not go on | ACM/MCM and HPL cladding, timber cladding, timber balconies, buildings under external wall remediation | Governed by structural survey, building control and fire strategy instead | Interim Product Specification clause 5.8 |
The difference that changes the arithmetic: no export payment
This is the point most comparisons skate over, and it inverts the usual advice about sizing. On a rooftop system, surplus generation still has a value, so oversizing is close to free: what you do not use, you sell. On a plug-in kit there is no buyer. Every kilowatt-hour the panels make while nobody is home leaves the house and earns nothing.
The consequence is that the two systems are optimised in opposite directions. A rooftop array wants maximum annual yield — south, tilted, as many panels as fit. A plug-in kit wants maximum overlap with your consumption, which is often east or west facing and vertical, because that spreads a smaller output across the morning and afternoon when a household is more likely to be drawing power. DESNZ’s own illustrative figures show the pattern: the 30°-tilted southern case generates far more but only 62% of it is used at home, while the vertical east/west case generates much less and uses 72%. The gap in saving is much narrower than the gap in generation.
It also means the honest question for a plug-in buyer is not “how much will it make” but “how much will I be using at eleven in the morning in April”. Our savings calculator is built around that, and why plug-in solar cannot get SEG payments sets out the certification reasoning in full.
The difference nobody prices: permanence
A rooftop array is a fixture. It is fixed to the roof covering, it is wired into the consumer unit, it comes with a certificate tied to the address, and it transfers with the property. That is a strength if you are staying and a liability if you are not — and it is why a rooftop quote includes scaffolding, a structural check, an installer’s certificate and, in England, notifiable electrical work under Part P.
A plug-in kit is expressly the opposite. The specification requires that any attachment method be reversible and non-permanent and that it must not compromise the structural integrity, fire performance or weatherproofing of the building. The whole product is designed so that removing it leaves nothing behind — which is exactly what a landlord, a freeholder or a managing agent needs to hear before granting consent. See plug-in solar for renters and plug-in solar in a leasehold flat.
What each one costs, and what we will not tell you
We publish prices only from our own dated survey. For plug-in kits, the DESNZ impact assessment records European 800 W systems as available for around £400 to £600 excluding installation or other ancillary costs, and the department’s illustrative annual saving is up to about £110. For rooftop, [VERIFICAR: our own dated UK price survey for a typical 4 kWp MCS rooftop installation, and a current SEG tariff range from Ofgem’s published supplier list] — we will not repeat installer marketing figures here.
The structural point stands without a price, though. A rooftop system’s fixed costs — scaffolding, survey, certification, a day or two of labour — are the same whether the array is 3 kW or 5 kW, so the cost per installed watt falls sharply as the system grows. A plug-in kit has almost no fixed cost and almost no economy of scale, because you are capped at 800 W of output. That is why the sensible framing is not “which is better value per watt” (rooftop, usually, by a wide margin) but “which of these can I actually buy”. See how much a plug-in solar kit costs and the payback period.
The case where the answer is “neither, yet”
If your household is empty from eight until six, has no timed loads you can shift, and no immersion heater or EV to soak up midday generation, a plug-in kit will export most of what it makes for nothing and the payback will be poor. Fixing the demand side first — a timer on the dishwasher, a smarter tariff, moving the washing to daylight hours — costs nothing and improves the case for either system. See how to use more of your solar at home and the best electricity tariff for plug-in solar.
And if the roof is genuinely good but the budget is not there yet, buying a plug-in kit is not a stepping stone to a rooftop system: none of the hardware transfers, and the two are separately certified. It is a different purchase, not a first instalment.
Frequently asked questions
Can I have a plug-in kit and a rooftop array at the same time?
That is a question for your DNO, not for us. Both are notifiable generation on the same installation, and G98 currently limits plug-in devices to one per household. Notify honestly and let the operator hold the complete picture — see how to fill in a G98 notification.
Does a plug-in kit need an electrician and a rooftop system not?
The other way round. A compliant plug-in kit plugged into a sound existing socket needs no electrician; a rooftop system is fixed-wiring work throughout. But anything on your side that touches the consumer unit, a circuit or the roof — including adding an outdoor socket for the kit — is work for a competent electrician working to BS 7671. See do I need an electrician.
Is a plug-in kit five times worse than a 4 kW roof?
In annual generation, roughly — it is about a fifth of the capacity. In annual saving, less than that, because the plug-in kit’s smaller output is used at home at a higher rate while a large rooftop array exports a great deal of its midday peak at a low SEG rate.
Will a plug-in kit work in a power cut?
No, and nor will a standard rooftop system. Both use grid-following inverters that shut down when the mains disappears. Backup requires a battery and specific switchgear — see home battery storage.
Which one is quicker?
A plug-in kit is an afternoon’s work plus a ten-minute notification. A rooftop installation involves a survey, a quotation, scheduling, scaffolding and certification. That difference is real, and for some households it is the whole decision.
Primary sources.
SI 2026/848 (legislation.gov.uk) ·
DESNZ, Plug-in Solar Device Interim Product Specification version 2.0, July 2026 (PDF) ·
DESNZ, plug-in solar consultation and government response ·
Ofgem, Smart Export Guarantee ·
Energy Networks Association, find my network operator.
General information, not legal or electrical advice. Anything touching fixed wiring, the consumer unit or the roof should be assessed by a competent electrician working to BS 7671 — see our editorial methodology.