TL;DR
Most UK homes need 8–12 panels, which is a 4–6 kWp system. Smaller households land at 6–8 panels; larger homes with EVs or heat pumps want 12–20. Modern 470W N-type panels mean you need fewer than you’d think.
Size to your actual bill, not your roof. Oversized arrays just export cheap. The calculator below gets you to a ballpark kWp in about 60 seconds.
Short answer for a UK home: 8–12 panels covers the typical 3—4-bed family. That’s a 4–6 kWp system using modern 470W panels, taking up roughly 18–26 m² of roof. The right number depends on three things — your annual kWh usage, where in the UK you are, and how much usable roof you have.
One caveat worth stating up front: don’t fill the roof. An installer who quotes “as many panels as will fit” is usually quoting for their margin, not your payback. Oversized arrays export surplus at 5–15p/kWh when they could have offset imports at 27–28p. Match the array to the bill. The savings calculator does the maths for you.
The short answer by property type
The table below is our back-of-envelope starting point before a site survey. It assumes 440–480W N-type panels and UK average generation of roughly 900 kWh per kWp per year.
2-bed flat or terrace — around 3 kWp
Typical annual usage sits at 2,400–3,000 kWh (close to the Ofgem Typical Domestic Consumption Value for a low-usage home). A 3 kWp system means 6–7 panels across roughly 12 m². With a 5 kWh battery, expect 50–65% bill reduction. Without a battery, payback stretches past 11 years because most of the midday generation exports while the flat is empty.
3-bed semi — around 4 kWp
The archetypal UK install. Annual usage typically runs 3,500–4,200 kWh, matching the Ofgem medium TDCV band. A 4 kWp array needs 8–10 panels and about 18 m² of south, east or west-facing roof. Pair with a 9.5 kWh battery and you’re looking at 60–75% bill savings and 6–8-year payback.
4-bed detached — around 5–6 kWp
Usage jumps to 4,500–6,000 kWh for families with teenagers, hot tubs, home offices or induction cooking. 5–6 kWp means 11–13 panels, often split across two roof planes. A 13 kWh battery fits this household well, and hybrid inverters at 5 kW keep you just inside the G98 3.68 kW single-phase limit once you apply power-export limiting (more on that below).
5+ bed, EV or heat pump home — 6–10 kWp
Heat pumps can double an annual bill to 7,000–10,000 kWh. An EV adds another 2,000–3,500 kWh depending on miles. You’ll want 13–20 panels and a 13–20 kWh battery, and you’ll almost certainly need a G99 DNO application rather than the under-the-wire G98 route. Don’t let that scare you — we handle the paperwork.
The actual sizing formula
Rules of thumb are fine for a first pass. For real numbers, use the formula we run at survey.
Start with your annual kWh usage
Grab your latest electricity bill (not gas). The figure you want is total kWh consumed over the last 12 months. The Ofgem Typical Domestic Consumption Values put a low-usage home at ~1,800 kWh/year, medium at ~2,700 kWh/year and high at ~4,100 kWh/year, though these are lower than real-world numbers for heat-pump or EV households. Add 2,000–3,500 kWh for an EV, or double your current electricity bill for a heat pump.
UK generation per kWp by region
A south-facing 1 kWp array generates around 900 kWh/year in the UK average. The south coast pushes 950–1,000 kWh/kWp/year; Yorkshire and the Midlands land at 850–900; northern Scotland drops to roughly 800. The PVGIS tool from the EU’s Joint Research Centre gives accurate postcode-level figures. East and west roofs lose about 10%; north-only loses 30–40%.
Roof area and panel count
Divide your target kWp by the panel wattage to get panel count. A 470W panel measures roughly 1.76 m × 1.13 m, so each one takes about 2 m². A 4 kWp array on 470W panels is 9 panels and roughly 18 m² of usable tile. Leave 40 cm edge setbacks and avoid skylights, vents and chimneys when you’re eyeballing the fit.
Worked example: a family of four using 4,200 kWh/year in Sussex wants to offset 70–80% of their bill. Target generation = 3,000 kWh. At 950 kWh/kWp, that’s 3.2 kWp — call it 3.5 kWp on 470W panels, so 8 panels and about 16 m². A battery lifts self-consumption enough to hit the target. See our roof suitability guide for the tile, pitch and structure checks.
Size your system
The calculator takes your usage and roof and returns a target kWp, annual generation, payback period and 25-year savings. The same engine runs every real quote we issue.
Estimated annual savings
£1,180/yr
That's £98/mo back in your pocket
Includes ~£87/yr export income from surplus sold to the grid
£11k
System cost
0% VAT
~8yrs
Payback
£32k
25-yr profit
Your 25-Year Savings Journey
See when your investment pays for itself
Estimate based on UK average solar yield and current electricity costs. Actual savings depend on roof orientation, shading, and usage. Book a free survey for a precise quote.
How to read your electricity bill for kWh
Ignore the pound signs for a minute. The number you need is total kWh over 12 months. Here’s where to find it.
- Unit rate. Usually printed near the top of the bill at something like 27p/kWh. Tied to the Ofgem default tariff cap unless you’re on a fixed deal.
- Standing charge. Around 50–60p a day whether you use power or not. Solar doesn’t remove this; only switching supplier or going off-grid does.
- Day and night readings. Economy 7 bills split usage into peak and off-peak. Add both to get your annual total. If you’re on an Octopus TOU tariff (Go, Flux, Cosy, Intelligent), your bill breaks consumption into named rate periods instead.
- Smart meter in-home display. The cheap plastic box on the worktop shows kWh live. Check the weekly or monthly total for a reality check against the bill. Multiply a summer average by 12 and you’ll underestimate — winter usage is usually 30–50% higher.
No smart meter? Read the mechanical meter today, read it again in a week, subtract. Multiply by 52. It’s rough but usable. You’ll also want a smart meter anyway before you can claim the Smart Export Guarantee — the SEG scheme requires SMETS2.
Why bigger isn’t always better
Two reasons to resist the urge to fill the roof: one regulatory, one economic.
The G98 3.68 kW single-phase limit
Under Engineering Recommendation G98, a single-phase home can install up to 3.68 kW of inverter export (16A) without getting prior DNO approval — you just notify them within 28 days of commissioning. Go bigger and you’re on the G99 track: full application, 45-65 working days, sometimes a connection upgrade fee. G99 is absolutely fine (we file them every week), but plan for it.
Practical trick: pair a 5–6 kWp DC array with a 3.68 kW export-limited hybrid inverter. You stay inside G98, but you get more yield in low light and charge the battery faster. MCS approves this pattern and most modern inverters (GivEnergy, SolaX, Fox ESS, Sigenergy) support export limiting natively.
Export waste economics
Every kWh you self-consume is worth your import rate — 27–28p today. Every kWh exported earns 5–15p through SEG. A 10 kWp array on a home that uses 4,000 kWh/year will push 6,000+ kWh a year to the grid at a fraction of its real value. You’re effectively buying cheap electricity for the neighbourhood. Don’t.
Sizing for EV owners
Rule of thumb: add 2–3 kWp to the base sizing for each EV. An average UK driver covers about 7,500 miles a year and a modern EV does 3.5 miles per kWh, so roughly 2,150 kWh/year per car. A 3 kWp uplift at UK average generation covers it with a bit of slack.
If you’re on Octopus Intelligent Go (7p/kWh overnight), you’ve got two ways to charge: solar surplus during the day or cheap-rate at night. A sensibly sized array plus a 9.5–13 kWh battery lets you mix both. Don’t oversize chasing the idea of a fully solar-charged car — UK winter sun simply won’t fill the battery most days. The maths works better if you split the job between solar and off-peak import.
Sizing for heat pump homes
A typical UK air-source heat pump swallows 4,000–6,000 kWh/year just for heating and hot water, on top of the 3,000–4,000 kWh the home already used for lights and appliances. Total annual draw often doubles. You’ll want 6–10 kWp of panels and a 13–20 kWh battery.
Reality check: the heat pump runs hardest in December and January when the panels generate 20–25% of their summer output. Your solar won’t cover winter heating. What it does do is offset the 8–9 months when the pump runs for hot water and shoulder heating, plus cover summer hot-water cylinder reheats entirely. Check the Energy Saving Trust’s solar guide for the regional generation tables before committing.
Sizing with a battery
Batteries change the sizing problem. Without one, you want the array to roughly match daytime demand. With one, you can comfortably oversize the array by 10–20% because the battery soaks up the surplus for evening use.
The battery itself should be sized to your evening load, not your panel count. Add up your kWh use between roughly 4pm and 11pm. That’s your target battery capacity. A 3-bed semi typically lands on a 9.5 kWh battery; a 4-bed detached with evening cooking, TV and an EV trickle on 13 kWh; a heat-pump home on 16–20 kWh. More on chemistry, warranties and tariff arbitrage on our battery storage page.
Common sizing mistakes UK installers see
We survey a lot of roofs. The same five errors crop up repeatedly. If your quote has any of these, push back.
- Sizing on roof area alone. A 32 m² south-facing roof can hold 16 panels — doesn’t mean you need 16. If the household uses 3,000 kWh a year, a 10-panel array and a small battery will earn you more money than the 16-panel version that exports half its output.
- Quoting for older 370W or 400W panels. Some installers are still shifting 2022-era stock. Same roof area gives you 25% less generation. Ask for 440–480W N-type as the default.
- Ignoring shading on the design. A chimney that throws a stripe across two panels between 10am–12pm costs real kWh. We model this at survey and either drop the shaded row, shift the layout, or use optimisers. If the quote doesn’t mention shading, it hasn’t been properly designed.
- Specifying an undersized inverter with no headroom. A 3.68 kW inverter on a 4 kWp array and no room for a future battery means replacing the whole inverter later. Spec a 5 kW hybrid from day one — it’s about £300 extra and saves £1,200 down the line.
- Matching panel count to household size, not usage. “Four people = 16 panels.” Utter nonsense. Two low-usage adults can be a 3 kWp home; a single heat-pump enthusiast can need 8 kWp. The kWh is the input, not the headcount.
- Forgetting the EV or heat pump that’s coming next year. You’re installing a 25-year asset. A 2-year look-ahead on future load adds 2–3 kWp to the array and pays back inside the warranty window. We ask about this at every survey.
Frequently asked questions
What if my roof is smaller than the ideal size?
You fit what the roof will take and accept a slightly lower bill saving. A 2.3 kWp array on 12 m² of usable roof still knocks £350–£500 a year off a typical bill. We'd rather install 5 well-placed 470W panels than squeeze 9 shaded ones onto the same tiles.
Can I add solar panels later?
Yes, if the inverter and cabling were specced for it at install. A hybrid inverter sized at 5 kW with headroom lets you bolt on another 1–2 kWp of panels (and a battery) down the line. If you're weighing cash now, start with the panels and add the battery in year 2 or 3. Do mention future plans at survey — retrofitting an undersized inverter costs roughly £900–£1,600.
Does a mix of roof orientations work?
Fine, and often better. An east/west split spreads generation across the day so more of it lands when you're actually using power. You lose around 10–15% of annual output compared with pure south, but self-consumption typically climbs. Our view: don't refuse a split-roof quote just because someone told you south-only.
Do optimisers or microinverters change the sizing?
They don't change how many panels you need, but they do rescue output when part of the roof shades. SolarEdge optimisers or Enphase micros let one shaded panel drop without dragging the whole string down with it. Useful on chimney-shadowed or tree-edge roofs. They add roughly £500–£1,200 to a typical install.
How does shading affect sizing?
Shade is the silent killer of string inverters. Two hours of chimney shadow on one panel can cost 10–20% of daily generation without optimisers. We model shading at survey with Suneye or a photo-sun-path tool and either upsize the array, drop the shaded row, or switch to panel-level electronics. Never accept a quote without a shade analysis.
What wattage panel should I ask for?
Modern N-type panels at 440–480W are the sweet spot in 2026. AIKO, REC, Longi and JA Solar all sit here. Don't go below 400W — you're just paying more for scaffolding and labour to mount more tiles. Higher wattages cut the panel count on tight roofs and leave headroom for a battery later.
Should I size for my current bill or my future bill?
Future, within reason. If you're getting an EV in the next two years, add 2–3 kWp to today's number. Heat pump coming? Plan for 6–10 kWp. Don't size for a hypothetical EV you may never buy, though — oversized arrays export at 5–15p/kWh instead of offsetting 27p, so the over-spend never comes back.
Want your exact panel count?
We’re an MCS-certified installer covering Surrey, Kent, Sussex, Essex, Berkshire, London, Bristol and Yorkshire. Every survey returns a panel-by-panel roof plan, a kWh generation estimate tied to your postcode, and a payback projection. Have a read of our ROI guide, check which panels we’d actually install, or see how the installation itself runs.
