4kW vs 6kW Solar System — Which Size Do You Need?

Last updated: July 2026 · Costs and savings are estimates — get quotes for your own roof

The single biggest decision when going solar isn't the brand of panel — it's the size of the system. In the UK the two most quoted sizes are 4kW (around 10 panels) and 6kW (around 14–15 panels). The right choice depends on your electricity use, your roof space, and whether you're planning a battery, heat pump or electric car. Here's how the two compare in 2026.

4kW vs 6kW at a glance

	4kW system	6kW system
Panels needed	~9–10 panels	~14–15 panels
Roof space	~20 m²	~29 m²
Typical installed cost (2026)	£5,500 – £8,000	£8,000 – £11,000
Estimated annual output	~3,400 kWh	~5,100 kWh
Suits annual usage of	Up to ~3,500 kWh	4,000 kWh+
Estimated annual benefit*	~£650 – £750	~£900 – £1,050
Estimated payback	~8–10 years	~9–11 years

*Estimates combining bill savings (electricity valued at the Jul–Sep 2026 price cap of ~26.1p/kWh) and export income at a ~15p/kWh SEG rate, assuming typical self-use. Actual results depend on your roof, location and usage patterns — these are not guaranteed savings.

How the savings actually work

Every kWh of solar you use yourself replaces electricity you'd otherwise buy at around 26.1p/kWh under the July–September 2026 price cap. Every kWh you don't use gets exported and paid via the Smart Export Guarantee — typically 12–15p/kWh from major suppliers, with some exclusive tariffs paying 20p+ (see our SEG rates comparison).

That gap between 26p saved and ~15p earned is the key to sizing. A 4kW system on a typical home means you self-use a bigger share of what you generate, so each panel works harder for you. A 6kW system generates more overall, but a larger slice gets exported at the lower rate — unless you can soak it up with a storage battery, an EV charger or a hot-water diverter.

When a 4kW system is the right call

A 4kW array suits most 2–3 bedroom homes using up to around 3,500 kWh a year. It's the cheaper ticket in — from about £5,500 installed at 0% VAT (which applies until 31 March 2027) — and because more of its output gets used in the home, the payback period is often slightly shorter than a bigger system. If your roof is small, shaded or split across aspects, 4kW may also simply be what fits.

When 6kW makes more sense

Go bigger if any of these apply: your household uses 4,000 kWh+ a year; you have (or plan) an electric car or heat pump; you're adding a battery; or you work from home and run appliances in daylight hours. The extra ~£2,000–£3,000 buys roughly 50% more annual generation, and the marginal cost per panel is lower because scaffolding, labour and the inverter are largely fixed costs. Homes in northern regions and Scotland generate a little less per panel, which can also tip the maths toward a larger array — see our guide to solar panels in Scotland.

Don't oversize without a plan for the surplus

The most common sizing mistake is fitting the biggest system the roof takes without thinking about where the electricity goes. On a sunny June day a 6kW system can generate 30+ kWh — far more than most homes use — and exporting the excess at 15p returns much less than avoiding a 26p import. A battery changes that equation significantly, and so does smart charging an EV from surplus solar. Also note that systems above 3.68kW output per phase need DNO approval (a G99 application), which your installer handles but can add a few weeks to the timeline.

What about grants?

There's no general UK grant that pays for panels outright, but 0% VAT until March 2027 and schemes like ECO4 for qualifying households do reduce the cost — our solar grants and funding guide covers what's actually available in 2026.