Buy the 800W-class kit if your site is anything other than perfect. The cheapest certified single-panel kit is £599 for 515W; the cheapest two-panel kit that saturates the 800W AC ceiling is £849 for 890W. That is 73% more panel for 42% more money, and the cost per watt falls from £1.16 to £0.95. The single-panel kit wins on one thing only: the smallest amount of money you can put at risk. On a shaded, east-facing or north-facing site, extra panel is the only lever you have, and the 800W route is not close.
"400W versus 800W" is the way the question gets asked, but there is no certified 400W kit in Great Britain. The real comparison is between the small single-panel end of the register — 460W and 515W — and the two-panel kits that fill the 800W AC ceiling. Everything below uses the same eight certified systems that drive the ledger.
800W versus 400W: the numbers side by side
| Small (single panel) | 800W-class (two panels) | |
|---|---|---|
| Cheapest certified kit | UKSOL Pro Compact 515W | UKSOL Pro Duo 890W |
| Price | £599 | £849 |
| Panel rating | 515W DC | 890W DC |
| Cost per watt | £1.16 | £0.95 |
| Fills the 800W AC ceiling? | No | Yes |
| Ledger payback range | 5.4–8.6 yrs | 7.7–12.1 yrs |
| Space needed | One panel position | Two panel positions |
| Best for | Small balcony, tight budget, trial run | Shaded or poorly oriented sites, main purchase |
The payback row looks decisive for the small kit and it is the least trustworthy row in the table. It divides each price by the government's £70–£110 band for an 800W system, applying that band unchanged to a 515W panel that will not generate as much. It is a ranking of money at risk, not a forecast. The methodology, and its limits, are set out in the payback guide.
Why 800W is the number that matters
800W is the AC output ceiling set by SI 2026 No. 848, measured at the microinverter. It is not a limit on panels. That is why kits carrying 890W, 1,030W and even 1,260W of panel are certified: the inverter clips its output at 800W AC on the handful of occasions when the array could exceed it.
The practical reading is that panel wattage above 800W is not wasted — it is insurance against the conditions in which you actually live. British generation is dominated by diffuse light, not clear noon sun. An 890W array spends the overwhelming majority of the year nowhere near the ceiling, which means almost every extra watt of panel converts into real output.
Which should you buy for your site?
Match the kit to the surface, not to a spreadsheet.
- Unshaded, south-facing, well-tilted. The best case, and the one case where a smaller kit genuinely holds up. You will approach the ceiling on good days, so extra panel earns less. The 515W at £599 is defensible here.
- East or west facing. Buy the panel. Output is spread thinner across the day and you will rarely trouble the 800W ceiling, so the 890W at £849 converts almost all of the extra capacity into electricity.
- Partly shaded — a parapet, a neighbouring block, trees. Buy more panel again, and consider the Pro Max 1260W at £989. Extra area is the only lever you have against shade, and at £0.78 per watt it is the cheapest capacity on the board.
- North facing, no alternative. Neither. This is the case where we would tell you not to buy at all rather than sell you a smaller version of a bad idea.
- A garden or yard. Ground-mount, and take the Duo 890W at £849 over the 460W at £699. The smaller ground-mount kit is the worst value on the register at £1.52 per watt.
The economics of the second panel
The clearest argument for going bigger is what the second panel actually costs. The microinverter, the mounting frame, the cabling and the shipping are broadly the same whether you hang one panel or two. The step from the 515W kit at £599 to the 890W kit at £849 is £250 for 375 extra watts — about £0.67 per marginal watt, against £1.16 per watt for the first kit.
| Step | Extra cost | Extra panel | Cost per extra watt |
|---|---|---|---|
| 515W → 890W | £250 | 375W | £0.67 |
| 890W → 1,030W | £50 | 140W | £0.36 |
| 1,030W → 1,260W | £90 | 230W | £0.39 |
Every upgrade step is cheaper per watt than the kit you started with. The £50 step from 890W to 1,030W is the standout: 140 extra watts of panel for the price of a weekly shop. If you have the space and you are already spending £849, spending £899 is close to free capacity.
When the small kit is genuinely right
Three honest cases:
- You only have one panel position. A narrow balcony rail, a single unshaded wall section. Two panels will not fit, and that settles it.
- You are testing the idea. £599 is the smallest lawful bet available. If your site turns out to be worse than you hoped, you have lost £250 less than the person who bought the Duo.
- You are renting short-term. One panel is easier to mount reversibly, easier to get consent for, and easier to move. See balcony solar for renters and flat owners.
What is not a good reason: "I only need a bit." Plug-in solar at any size covers a fraction of a household's use — the government's own estimate is up to 20% for a full 800W system. Buying smaller does not right-size it to your needs; it just generates less.
The verdict
For most buyers, the UKSOL Pro Duo 890W at £849 is the right kit: enough panel to fill the ceiling, the cheapest kit that does so, and a mounting choice between hybrid and ground-mount at the same price. Step up to the Pro Plus 1030W at £899 if you have the space, because £50 for 140W is the best value move on the board. Take the Pro Compact 515W at £599 only if you are space-limited or deliberately testing the water.
The full ranked list, including the kits still awaiting certification, is in best plug-in solar kits UK 2026.
Sources and further reading
- SI 2026 No. 848 — the 800W AC ceiling, measured at the microinverter.
- GOV.UK — Households can save as plug-in solar panels come to market — the £70–£110 saving band and the "up to 20% of an average home's electricity" figure.
- ENA Connect Direct — the type-test register behind every kit listed here.
- Energy Saving Trust — independent guidance on orientation, tilt and shading in the UK.
- Retailer list prices, checked manually. Not an affiliate feed.
Frequently asked questions
Should I buy an 800W or a 400W plug-in solar kit?
An 800W-class kit, unless you are limited to a single panel position or deliberately spending as little as possible. The cheapest kit that fills the 800W ceiling is the UKSOL Pro Duo 890W at £849, against £599 for a 515W single panel — 73% more panel for 42% more money.
Is there a certified 400W plug-in solar kit in the UK?
Not at exactly 400W. The smallest certified systems on the ENA register are the UKSOL Pro Compact 460W ground-mount at £699 and the Pro Compact 515W hybrid at £599. The 515W is much the better value of the two.
Why do 800W kits have more than 800W of panel?
Because 800W is a cap on AC output at the microinverter, not on panel area. A 890W or 1,260W array feeding an 800W microinverter is compliant, and the inverter clips output on the rare occasions the array could exceed the ceiling.
Does a bigger kit pay back more slowly?
On the simple price-divided-by-saving model, yes, because it costs more. That model applies the same annual saving to every kit, which is not realistic. On real generation, larger kits close most of the gap, and on shaded or poorly oriented sites they will beat the smaller kit outright.
What is the best value upgrade?
Stepping from the 890W kit at £849 to the 1,030W kit at £899. That is £50 for 140 extra watts of panel — £0.36 per watt, against £1.16 per watt for the cheapest kit on the board.
How much space does a two-panel kit need?
Two panel positions rather than one, whether side by side on a balcony rail, across a wall, or on a ground frame. If you only have room for one panel, the decision is made for you.
Will a bigger kit export more to the grid for nothing?
Potentially, yes, and it is a fair concern. A plug-in device is not metered for export, so surplus is unpaid. That is an argument for shifting appliance use into daylight hours rather than for buying a smaller kit — a smaller kit generates less at every hour, not just the surplus hours.
Which kit should I buy for a shaded balcony?
The largest you can fit — the Pro Max 1260W at £989 if space allows. Extra panel area is the only effective response to shade, and at £0.78 per watt it is the cheapest capacity on the certified list.