The average new UK domestic system is around 4.8 to 5.2 kWp, up from about 3.6 kWp in 2021. For most three-bedroom houses that means somewhere around 4kW to 5kW.
That is the average. Here is how to work out yours.
Figures from the Microgeneration Certification Scheme (MCS), last reviewed 6 Nov 2026.
Start with your consumption, not your roof
Find your annual electricity use in kWh. It is on your bill, or in your online account. A typical UK household uses somewhere between 2,000 and 4,000 kWh a year, though a home with an electric car or a heat pump uses far more.
As a very rough guide, 1kW of panels generates around 850 to 1,000 kWh a year in the UK, depending on where you are and which way the roof faces. Southern England does better than northern Scotland, but the difference is smaller than people assume.
So a 4kW system produces roughly 3,400 to 4,000 kWh a year. On paper that covers a typical household’s entire annual consumption.
It does not work like that in practice, because the generation and the consumption happen at different times. Which brings us to the real question.
The real question is how much you will use
You will not use all of it. Solar generates in the middle of the day; most households use most of their electricity in the morning and evening.
Without a battery, a typical household uses well under half of what it generates and exports the rest. A unit used saves you about 26.32p. A unit exported earns roughly half that.
So sizing is a trade-off:
- Size to your daytime usage and you use nearly everything you generate, but you leave roof space and cheap generation on the table.
- Fill the roof and you generate more in total, but a larger share goes out at the export rate.
Why filling the roof is usually right anyway
Because of how the costs work.
Scaffolding, certification, network paperwork and most of the labour cost roughly the same whether you fit eight panels or sixteen. Those fixed costs get spread across however many watts you install. Each extra panel on an already scaffolded roof is close to just the cost of the panel and its mounting.
So the marginal panels are the cheap ones, even if they mostly export. This is why installers usually suggest more rather than less, and on this occasion it is generally sound advice rather than a sales tactic.
Where it stops being right: if you have very low daytime use, no battery, no plans for an EV or a heat pump, and no intention of changing tariff, then generating a lot you will only ever export at half value is a weaker case.
The 3.68kW question
You will see 3.68kW quoted a lot, and it is not a coincidence. It is the threshold below which a system can connect to the network without prior permission.
An installer designing to 3.68kW is avoiding an application and a wait. One proposing 5kW is going through the longer route. Ask which you are being offered, because it is a real difference that price alone will not reveal. See grid connection.
Things that change the answer
- A battery. Raises the share you use, so a larger system makes more sense.
- An electric car. A large, flexible load. Size up.
- A heat pump. A large load across much of the year. Size up.
- A time-of-use export tariff. Changes what exported units are worth.
- Shading. Reduces output from affected panels and may change the layout.
- Roof faces. Panels split across two or three faces generate across more of the day, which can suit usage better than everything on one south roof.
What to ask for in a quote
- The system size in kW, not the number of panels. Panel wattages vary too much for panel count to tell you anything.
- The estimated annual generation in kWh, worked out for your roof specifically.
- The assumed self-consumption percentage, and the reasoning behind it.
- Whether it is a G98 or G99 connection.
- What the next size up would cost, and what it would add in generation.
That last question is the most useful one, because it tells you the marginal cost of the extra capacity rather than the average cost of the system.