Plug-in Solar or Rooftop Solar? 800 VA Versus a Designed System

They are not really competing products. Plug-in solar offsets background daytime load. A designed system changes your electricity bill.

Plug-in solar is capped at 800 VA into the installation, cannot include a battery, and is currently limited to one device per household. In exchange it needs no roof work, no scaffolding, no design, no notifiable electrical work and no installer — which is exactly why it exists.

A rooftop installation under BS 7671 Section 712 has none of those ceilings and all of that work. If you own the roof and intend to stay, it will do far more. If you rent or live in a flat, it is not available to you at all.

This is the thing people underestimate. The plug-in limits are product limits, not design decisions — you cannot engineer past them, and a compliant device physically will not exceed them:

• 800 VA maximum apparent power supplied to the installation, and 3.5 A maximum current.
• 2000 W maximum total PV module DC power — you may fit more panel than inverter, which helps in poor light, but the output is still capped.
• Up to 4 modules, no more than 2 in series, and no more than 120 V DC at the inverter inputs.
• One device per household under G98 as it stands, expected to move to one per power circuit.

The reason for the ceiling is the whole architecture: the device injects current into an existing final circuit that was never designed to have a source on it. That is safe at a few amps into a properly protected circuit. It stops being a plug-in product long before it becomes a meaningful generator.

Worth stating plainly because it is the single biggest limitation and it surprises people. The specification excludes plug-in battery systems and plug-in solar PV devices integrated with battery systems from its scope entirely, and requires the instructions supplied with a compliant device to carry a prominent warning that it is not intended to be connected to, operated with, or used in conjunction with a battery energy storage system.

Electrical Safety First are blunter still, advising against installing plug-in battery systems in the home at all — while noting that they are already available from online retailers, which is the point of the warning.

This matters more than the headline wattage. Without storage, everything the device generates must be used at the moment it is generated or exported for very little. Storage is what turns solar generation into evening usage, and it is only available on the designed route.

Useful framing for a customer conversation. 800 VA is roughly the background load of an occupied house on a normal afternoon: a fridge-freezer cycling, a router, a few devices on standby, some lighting, a television.

The device supplies whatever is drawing load on that circuit first. Anything left over flows back through the board to the rest of the house, and only what the house cannot use passes out through the meter. So the value is driven by self-consumption during daylight, not by generation.

Which means the same kit is worth substantially more to someone at home during the day than to someone out from eight until six. It is one of the few products where the customer’s routine matters more than the specification.

And it does nothing in a power cut. The inverter is grid-following: it locks onto the supply already present and cannot create one. Pull the main switch and the device stops.

• Capacity. Plug-in: 800 VA, hard ceiling. Designed: sized to the roof and the demand.
• Storage. Plug-in: prohibited. Designed: available.
• Who installs it. Plug-in: the occupier. Designed: a competent electrician, with certification.
• Building work. Plug-in: none — mounting must be reversible and must not compromise the structure, fire performance or weatherproofing. Designed: roof penetration, scaffolding, structural check.
• Notification. Plug-in: G98 connect-and-notify within 28 days. Designed: G98 or G99 depending on size, plus notifiable work under Building Regulations and a BS 7671 certificate.
• If you move. Plug-in: unplug it and take it — though both the disconnection and the reconnection are notifiable. Designed: it stays with the building.
• Where it can go. Plug-in: not on ACM, MCM or HPL cladding, timber cladding, timber balconies, or buildings under external wall remediation. Designed: assessed as part of the design.

Plug-in solar is the right answer for renters, for flats and maisonettes, for anyone who cannot alter the building or does not intend to stay, for people who want to try solar without committing several thousand pounds, and for households with a genuine daytime load and a sunny bit of ground or wall.

A designed system is the right answer for owner-occupiers staying put, for anyone who wants meaningful export income, for anyone who wants storage, for households with high consumption or an EV, and for anyone whose roof is suitable and unshaded.

The dishonest version of this page would push everyone towards the bigger job. Plug-in solar is genuinely a good product for the people it was designed for — and the ones it does not suit will work that out from the 800 VA figure without needing to be told.

Starting with plug-in and moving to a designed installation later is a perfectly reasonable path, and the plug-in kit does not become worthless — it can move to another property, or to a shed or outbuilding on the same one, subject to the same notification rules.

What you should not do is run both. If a designed system is going on the roof, you already have the capacity the plug-in device existed to provide, and keeping it brings the one-per-household limit, the notification duties and the RCD considerations into play for a marginal gain.

For electricians: a customer who has bought a plug-in kit has already self-identified as interested in generation, has usually discovered its limits within a season, and has an installation you have possibly already assessed. The plug-in rules are on the hub page, and a designed system is certified as a Section 712 installation in the usual way.