Solar panel system cost by size: 3kW to 6kW
The single biggest thing that moves a solar quote is the system size — but "bigger" and "better value" are not the same thing. This page lays out the honest cost, generation, saving and payback for each common size, side by side, so you can see what your money buys at 3kW, 4kW, 5kW and 6kW before an installer ever knocks.
Solar system cost by size — the 2026 comparison
Every figure below is a range, dated as at July 2026, with 0% VAT applied (in force to 31 March 2027). Costs assume a standard pitched roof with straightforward access; generation assumes a decent south-facing, unshaded roof at roughly 950 kWh per kWp a year. Panels are counted at about 440W each. These are planning figures to compare sizes — your own numbers are only firm after a survey.
| System size | Panels (~440W) | Roof area | Installed cost (0% VAT) | Annual generation | Typical annual saving | Indicative payback |
|---|---|---|---|---|---|---|
| 3kWp | ~7 panels | ~15–18 m² | £5,000–£6,000 | ~2,600–2,850 kWh | ~£300–£500 | ~10–14 years |
| 4kWp | ~9–10 panels | ~20–24 m² | £6,000–£8,000 | ~3,400–3,800 kWh | ~£400–£700 | ~8–13 years |
| 5kWp | ~11–12 panels | ~25–30 m² | £7,000–£9,000 | ~4,300–4,750 kWh | ~£550–£850 | ~8–12 years |
| 6kWp | ~13–14 panels | ~30–36 m² | £8,000–£11,000 | ~5,100–5,700 kWh | ~£650–£1,000 | ~8–12 years |
Two things jump out of that table. First, the saving grows with size but the payback barely changes — because cost and saving scale together. Second, and less obvious, the payback ranges all assume you actually use a good share of the generation at home; on a larger system that's the assumption that can quietly break. More on that below. For a general breakdown of what a system costs regardless of size, see our full solar panel cost guide.
Why cost per kWp falls as the system grows
Look at the cost column and the maths seems odd: a 6kWp system is twice the size of a 3kWp one, but nowhere near twice the price. That's because a solar install is only partly about the panels. A big share of any quote is fixed cost that barely changes with size — a day of scaffolding, the crew's time on site, the inverter, the DNO connection paperwork, and the electrical work to tie it into your consumer unit. Spread those fixed costs across more panels and the price per kilowatt falls.
In round numbers, you're paying somewhere near £1,700–£2,000 per kWp on a small 3kWp array, dropping toward £1,400–£1,700 per kWp by the time you reach 5–6kWp. That's why adding a few extra panels at the design stage is usually the cheapest solar you'll ever buy — the scaffolding is already up and the crew is already there. It does not follow, though, that you should max out the roof: extra panels are only worth having if the electricity they make is worth something to you, which brings us to the real question behind sizing.
When it's worth sizing up
Because the marginal panels are cheap, sizing up makes sense for a lot of homes — but only if you can turn the extra generation into avoided grid imports. A self-consumed unit avoids about 26p off your bill (the Ofgem price cap is 26.11p/kWh for July–September 2026), while a unit exported under the Smart Export Guarantee earns only about 12p (Octopus Outgoing, as at July 2026). So a unit you use is worth roughly double a unit you export. Sizing up pays when:
- Your annual electricity use is high — a bigger bill has more expensive units to displace.
- You're home during the day, work from home, or run daytime loads like a hot-water diverter, dishwasher or washing machine on a timer.
- You charge an EV in daylight or run a heat pump, both of which create a large, controllable daytime load — this is the strongest case for a 5–6kWp system and the shortest paybacks in the table.
- You plan to add a battery later, which stores midday surplus for the evening and lifts self-consumption from around 45–50% toward 80–90%.
For those households the extra panels earn near-full value, which is why high-usage and EV or heat-pump homes tend to see the best return on investment. If that sounds like you, size generously while the crew and scaffolding costs are shared across more panels.
When it's better to size down
The opposite is just as true, and it's the part pushy quotes skip. If you're out all day with no battery, most of what a large array makes will spill onto the grid at about 12p instead of saving you about 26p — so those extra panels earn barely half their potential and the payback in the table drifts toward its slower end. Sizing down, or holding at 3–4kWp, is the honest call when:
- Your electricity bill is modest (under roughly £900–£1,000 a year) — there's less to save, so a smaller, cheaper system keeps the payback sensible.
- The house is empty through the working day and you're not adding a battery.
- Your roof is small, steep, partly shaded, or split across awkward faces that limit how many panels sit in good sun.
- You'd rather spend less up front and add capacity or storage later once you see how the system performs.
There's no prize for the biggest array. The right size is the one where the generation matches the electricity you'll genuinely use, not the number of panels that fit on the tiles. If you're weighing the whole decision, our straight answer on whether solar panels are worth it sets out when the case is strong and when it isn't.
0% VAT applies whatever size you choose
One thing does not change with size: the tax break. Under VAT Notice 708/6, residential solar and battery storage — equipment and installation — carry 0% VAT until 31 March 2027, whether you fit a 3kWp array or a 6kWp one. From 1 April 2027 the rate reverts to 5%. On the systems in the table above, that 5% would add roughly £300 on a 3kWp install and up to about £550 on a 6kWp one if you buy after the deadline.
It's a real, dated saving worth building into your timing — but it isn't a reason to rush a bad decision. A poorly-oriented, heavily shaded or soon-to-be-sold roof still won't pay back, VAT break or not. Treat the deadline as a nudge to get your surveys and quotes done in good time, not to skip the survey and sign today. Separately, some suppliers forecast module prices rising 10–15% during 2026 after China removed a solar export VAT rebate in April 2026 — but that's a forecast about the panels alone, which are only one part of an installed price that is mostly labour and scaffolding, so it may move your quote far less than the headline suggests.
Match the size to your usage, not your roof
The temptation is to size a system by the roof — "how many panels will fit?" — when the number that actually matters is your annual electricity use. A rough guide: a lower user (around 1,800–2,500 kWh a year) suits 3–3.5kWp; the average UK 3-bed semi (around 2,900–3,500 kWh) suits 3.5–4.5kWp; and a high user, or a home adding an EV or heat pump, suits 5–6kWp. The Energy Saving Trust's reference system is about 4.5kWp, which is why 4kWp is the size most quotes default to. For the full method — turning your usage into a panel count — see how many solar panels do I need.
Once you've picked a size, cost is only half the sum — what matters is that cost against the annual saving, which is what gives you payback. A £7,000 system saving £550 a year pays back in about 13 years; the same £7,000 saving £750 a year is nearer 9. UK domestic solar generally lands in the 7–12 year range on panels warrantied for 25 years or more. Run your own numbers on the solar savings calculator, read the full detail of how much solar panels save, and see the honest working behind the solar panel payback period before you commit to a size.
Comparing quotes at the same size
Two installers can quote the same 4kWp system and land thousands of pounds apart, because "4kWp" only describes the panels — it says nothing about your roof, your electrics or the kit quality. When you compare quotes at a fixed size, look past the headline and check the parts that genuinely move the price: scaffolding and access, roof type and condition, inverter choice (a single string inverter versus microinverters or optimisers for a shaded or multi-facet roof), any consumer-unit or wiring upgrade to meet BS 7671, the DNO connection type (a G99 application above 3.68kW per phase adds admin and lead time), and whether a battery is bundled in.
Get two or three itemised quotes from MCS-certified installers — MCS is the standard that also unlocks your export payments — each RECC or TrustMark registered, with NICEIC- or NAPIT-registered electricians. A good quote breaks out equipment, scaffolding, electrical work and certification so you can compare like for like. Be wary at both ends: a suspiciously cheap quote usually skimps on kit or certification, while an inflated one may bundle extras you don't need. Under the RECC code an installer can take a deposit of no more than 25%, so treat demands for large upfront payments as a red flag, and walk away from anyone promising "solar panels for free" or calling themselves a "government-approved solar installer" — no such badge exists. A fair price is the one backed by proper certification and a system sized to your roof and your usage.