It makes more electricity than most homes use, and you will still get a bill
If you have a quote for a 4kW system in front of you, you want three answers: is the price fair, will it cover your electricity, and how long until it pays for itself. This guide gives you all three with the arithmetic shown, so you can hold any quote up against it.
A 4kW system costs £5,500-£7,500 fully installed in September 2026 and makes roughly 3,100-4,300 kWh a year, depending mostly on where you live. That is more than Ofgem’s typical household uses. But around 70% of it arrives between April and September, and much of it lands while you’re out, so most homes still buy around half their electricity from the grid.
Below you’ll find what to pay, what your roof will make month by month, payback on today’s tariffs, and whether 4kW is the right size. We also cover the 3.68kW inverter rule behind almost every 4kW design, which most guides skip.
- Cost: £5,500-£7,500 installed. The government’s 2025/26 median is about £6,800-£7,200. 0% VAT applies until 31 March 2027.
- Output: about 3,600 kWh a year in the middle of the UK. Expect 3,100-3,600 kWh in Scotland and 3,850-4,300 kWh on a south-facing roof in southern England.
- Payback: about 8-14 years without a battery. Your export tariff can move that by three years.
- Hardware: 9 x 450W or 10 x 400W panels on 20-24m² of roof, usually on a 3.6kW inverter so it connects under the simpler G98 grid rule.
- The catch: extra panels are cheap once the scaffolding is up. If your roof fits two more, the step towards 5kW often adds only £400-£700.
How much does a 4kW solar panel system cost in 2026?
You’ll pay £5,500-£7,500 for a 4kW system fully installed, including scaffolding, MCS certification and the grid paperwork. On an ordinary two-storey house with a pitched tiled roof, most quotes land around £6,500.
The best yardstick is government data. DESNZ figures for 2025/26, drawn from 265,682 MCS-certified installs, put the median at roughly £1,700-£1,800 per kW, or £6,800-£7,200 for 4kW. That median includes every London scaffold and awkward slate roof, so a simple job should come in below it.
| Source | System | Price | Per kWp |
|---|---|---|---|
| DESNZ median, 2025/26 | 4kW, all regions and roof types | £6,800-£7,200 | £1,700-£1,800 |
| E.ON Next | 6 panels, 2.91kWp, no battery | £4,995 | £1,716 |
| E.ON Next | 12 panels, 5.82kWp, no battery | £6,495 | £1,116 |
| Octopus Energy | Entry price, 2 panels, no battery (June 2026) | £6,663 | Not meaningful |
| Octopus Energy | 8 x 450W (3.6kWp) + 5kWh battery (June 2026) | £8,623 | Includes battery |
Look at the two E.ON rows. Doubling the system from 2.91kWp to 5.82kWp adds £1,500, which is about £515 for each extra kWp. Octopus charging £6,663 for only two panels tells the same story. The first kilowatt carries the scaffolding, labour and paperwork, and every kilowatt after it is cheap. From-prices assume a simple roof, so real quotes rise with complexity.
The panels are about a sixth of the bill. Scaffolding, labour and overheads cost much the same for six panels or twelve. What pushes a quote above the typical range is mostly access and the house itself.
| Extra | Typical addition |
|---|---|
| Slate or clay tile roof instead of concrete tiles | £300-£800 |
| Three storeys, or scaffolding over a conservatory or extension | £300-£800 |
| Consumer unit upgrade on an older fuse board | £300-£500 |
| Bird protection mesh around the array | £250-£400 |
| Microinverters or optimisers for a shaded or split roof | £500-£1,000 |
| London and South East labour rates | +10-20% |
0% VAT on home solar and batteries ends on 31 March 2027, then rises to 5%, adding about £325 to a £6,500 system. With 6-10 week lead times common, a quote signed by January leaves a safe margin.
Prices for every system size are in our solar panel cost guide, and the extras left off headline prices in hidden costs of solar panels.
How much electricity does a 4kW system produce?
A 4kW system makes about 3,600 kWh a year on a south-facing roof in the middle of the UK, roughly 900 kWh for every kW of panels. Location, roof direction and shade move that figure more than panel brand.
| Region | kWh per kWp | 4kWp per year |
|---|---|---|
| South West England | 1,000-1,080 | 4,000-4,300 kWh |
| London and South East | 960-1,040 | 3,850-4,150 kWh |
| Midlands and East of England | 920-1,000 | 3,700-4,000 kWh |
| Wales | 900-980 | 3,600-3,900 kWh |
| Northern England | 870-950 | 3,500-3,800 kWh |
| Northern Ireland and Scotland’s Central Belt | 830-900 | 3,300-3,600 kWh |
| Northern Scotland | 770-850 | 3,100-3,400 kWh |
These are planning estimates for an unshaded, south-facing roof at 30-40°, based on PVGIS modelling. The spread from Cornwall to the Highlands is about 30%, and lower install prices further north claw some back. An installer’s MCS estimate may sit 5-10% lower once it applies a shading factor.
East or west-facing panels make about 80-85% of the south-facing figure, and a split east-west array spreads generation across the day. North-facing roofs make 50-65%, which rarely justifies the spend. See north-facing roofs and the best roof angle.
A June day makes five times what a December day does. Around 15 kWh on a summer day covers a typical home twice over. Around 3 kWh in December covers well under half. Remember that if anyone pitches 4kW as the answer to winter heat pump bills.
Is 4kW the right size for your home?
4kW suits a household using roughly 2,000-3,500 kWh of electricity a year, with gas heating and no electric car. Size from the annual kWh figure on your bill or supplier app, not your number of bedrooms.
Ofgem cut its “typical” household electricity figure from 2,700 kWh to 2,500 kWh a year from 1 July 2026. On paper, a 4kW system now out-produces that typical home by around 40%. Timing, not the annual total, decides how much of it you use.
| Annual use | What a 4kW system does | Size to price up |
|---|---|---|
| Under 2,000 kWh | Exports most of its output, so payback leans on your export rate | 2.5-3kW, or 4kW with a 12p+ export tariff |
| 2,000-3,500 kWh | Covers most summer daytime use and cuts the bill by a third or more | 4kW |
| 3,500-5,000 kWh | Useful, but leaves a large gap from October to March | 5kW if the roof fits |
| 5,000 kWh+ | Undersized for an EV, heat pump or electric heating | 6kW+ with a battery and smart tariff |
Most sizing goes wrong by assuming today’s usage lasts. An electric car driven 7,000-10,000 miles adds roughly 2,000-3,000 kWh a year. A heat pump adds 3,000-5,000 kWh, mostly between November and February, when a 4kW array makes only about 14% of its annual output. If either is likely, price a 5-6kW system alongside the 4kW quote. See solar for a 3-bed house and a 4-bed house.
Why a slightly bigger system often costs less per unit
E.ON Next’s prices put the second half of a 5.82kWp system at about £515 per kWp, against a market median of £1,700-£1,800. Two extra 450W panels (0.9kWp) typically add £400-£700 to a quote and around 800 kWh a year in central England.
Even if every extra unit were exported at 12p, that’s about £97 a year, a 5-7 year payback and faster than the system as a whole. Undersizing to save £500 is usually a false economy if the roof has space. The exceptions: a low-usage home on a 3-6p export tariff earns far less from surplus, and beyond about 4.5-5kWp you’ll usually need a bigger inverter and a G99 application.
Savings and payback: the worked numbers
Without a battery, a 4kW system in central England pays back in about 8-14 years on September 2026 prices. Where you land depends on two things you control: how much of the output you use as it’s generated, and which export tariff you sign up to.
Take a 4.05kWp system costing £6,500 on a south-facing semi in the Midlands. It makes 3,700 kWh a year. The household uses 3,000 kWh and is out at work most weekdays, so it uses about 35% of the solar output as it’s generated. That’s 1,295 kWh it no longer buys. At the October 2026 price cap rate of 26.32p, that saves £341. The other 2,405 kWh goes to the grid. On a 12p export tariff, it earns £289. The total is £630 a year. Divide £6,500 by £630 and payback is 10.3 years.
Now change one thing. The same household stays with its current supplier and takes an open 6p export tariff. Export income halves to £144, and payback stretches to 13.4 years. The export tariff moved payback by three years, more than working from home would. Note which number each rate applies to: the unit rate to solar you use, the export rate to everything else.
| Household | Solar used at home | Export rate | Benefit per year | Payback |
|---|---|---|---|---|
| Out all day, open export tariff | 35% | 6p | £485 | 13.4 yrs |
| Out all day, supplier export tariff | 35% | 12p | £630 | 10.3 yrs |
| Home in the daytime | 50% | 12p | £709 | 9.2 yrs |
| Home in the daytime, best flat export | 50% | 15p | £764 | 8.5 yrs |
Every row assumes flat electricity prices, which is cautious, and leaves out one inverter replacement at year 10-15 (£600-£1,200, adding about a year). Panels carry 25-30 year performance warranties, so a decade or more of low-cost power follows payback. The 26.32p rate excludes VAT under a temporary cut to 31 March 2027; if 5% returns, each unit you use is worth 1.3p more.
Open tariffs that don’t need you to switch supplier pay about 3-6p per kWh, such as ScottishPower SmartGen at 6p. Tariffs tied to your import supplier pay 12-18p: Outgoing Octopus 12p (cut from 15p in March 2026), British Gas 15.1p, EDF 15-18p, and E.ON Next 17.5p where E.ON fitted the system. Compare the import rate too.
Run your own numbers in our Smart Export Guarantee calculator.
How many panels are in a 4kW system, and how much roof space?
Most 4kW systems in 2026 use 9 x 450W or 10 x 400W panels and need about 20-24m² of clear roof, including the edge gaps installers leave for wind loading and access.
| Panel rating | Panels | System size | Panel area |
|---|---|---|---|
| 400W | 10 | 4.00kWp | ~19m² |
| 430W | 10 | 4.30kWp | ~20m² |
| 450W | 9 | 4.05kWp | ~17m² |
| 485W | 8 | 3.88kWp | ~16m² |
| 500W | 8 | 4.00kWp | ~18m² |
A 4kWp system makes about the same energy whichever panels build it, so paying extra for efficiency only makes sense when roof space is the limit, such as a terrace with a chimney and roof window in the way. See how many solar panels you need and the best solar panels for homes.
Should you add a battery to a 4kW system?
For most 4kW homes a battery pays back no faster than the panels do, so add one for a reason, not by default. The strong reasons are an electric car on a cheap overnight tariff, heavy evening use, or wanting backup in power cuts.
Size the battery to evening and overnight use, not to the panels. A home on Ofgem’s typical 2,500 kWh uses about 7 kWh a day, often 4-5 kWh after dark. A 5kWh battery fits most 4kW systems, and a summer surplus of 8-12 kWh fills it. Go to 9-10kWh for overnight tariff charging, an EV or a heat pump.
Say a 5kWh battery adds £2,500. From April to September it moves about 700 kWh of solar you’d have exported at 12p into evening use worth 26.32p. That’s about £100. From October to March it fills overnight at around 7p on a smart tariff, shifting roughly 600 kWh. That’s about £115. Together, £215 a year. That’s a payback of about 12 years, against a typical 10-12 year battery warranty.
Intelligent Octopus Flux, the tariff that made batteries pay fastest, closed to new customers in March 2026. Fitted with the panels, a 5kWh battery costs £2,000-£3,000 and a 9-10kWh battery £3,500-£5,500. Later costs more: E.ON Next’s 2026 from-prices are £1,900 with a new install and £3,995 as a retrofit.
Ask for a 3.6kW hybrid inverter now. It usually adds £200-£450 and lets you add a battery later without replacing the inverter. It also keeps you inside the G98 grid limit: a 3.6kW solar inverter plus a separate 3kW battery inverter counts as 6.6kW, which needs your network operator’s approval before installation.
Compare models in our solar battery costs guide or test your usage in the solar battery calculator.
The 3.68kW inverter rule, and why your 4kW system has a 3.6kW inverter
Most 4kW systems use a 3.6kW inverter on purpose, and it costs you very little output. Inverters up to 3.68kW on a normal single-phase supply connect under the G98 rule: the installer fits the system, then notifies your Distribution Network Operator (DNO) within 28 days. Anything larger needs a G99 application, which must be approved before installation and can add several weeks.
Pairing 4.05kWp of panels with a 3.6kW inverter gives a ratio of about 1.1. UK panels rarely reach their full rating, so at 1.1-1.25 the output lost to “clipping” is usually 1-2% of the year’s generation or less, worth under £20 a year.
On a quote, divide total panel watts by the inverter’s AC rating: 1.05-1.25 is normal. A 4kWp array on a 5kW inverter triggers G99 for no extra generation, unless it’s a hybrid sized for a future battery. More in G98 vs G99 and export limitation.
0% VAT, grants and finance for a 4kW system
For most homeowners the only discount is 0% VAT, ending 31 March 2027. Funded solar exists, but it’s tied to income, benefits and a poor EPC rating.
- 0% VAT. Applied automatically when one company supplies and fits the system.
- ECO4. Can fully fund solar for households on qualifying benefits in homes rated EPC D-G. The scheme runs until 31 December 2026.
- Warm Homes: Local Grant (England). Funded upgrades, which can include solar, for low-income households in EPC D-G homes, delivered through councils.
- Warm Homes Loan Scheme. Government-backed, lower-interest loans for solar, batteries and heat pumps through approved lenders. Lenders began onboarding in August 2026, so check what’s live before taking other finance.
- Scotland. Home Energy Scotland stopped funding standalone solar PV in June 2024. Households on qualifying benefits should check Warmer Homes Scotland.
- Smart Export Guarantee. Needs an MCS certificate and a smart meter, so a cheaper non-MCS install forfeits every export payment.
Eligibility is covered in our solar panel grants guide and solar panel financing.
How to check a 4kW solar quote
A fair quote tells you exactly what goes on your roof and what it should make. Get three from MCS-certified installers, and ask each for these in writing:
- Panel make, model and count, plus total kWp. “4kW system” on its own tells you nothing about the kit.
- Inverter model and AC rating. If it’s above 3.68kW, who handles the G99 application and how long it adds.
- An MCS generation estimate in kWh a year, with the shading factor used for your roof.
- Scaffolding, bird mesh and consumer unit work, each marked included or excluded.
- A hybrid inverter, if there’s any chance you’ll add a battery.
- Workmanship warranty length and deposit protection, plus membership of a consumer code such as RECC or HIES.
- 0% VAT shown on the quote, not VAT added and then “discounted” away.
4kW solar panel system FAQs
How many kWh does a 4kW solar system produce per day?
About 10 kWh a day on average across the year for a south-facing system in central England. That ranges from around 15-16 kWh on a June day to about 3 kWh in December.
Will a 4kW solar system run a whole house?
Over a full year it can match or beat what a typical home uses, but not hour by hour. Without a battery, most homes use 30-50% of the output directly and still import from the grid in the evenings and through winter.
Can you go off-grid with a 4kW solar system?
Not for a normal UK home. December output of around 3 kWh a day is less than half of typical daily use, so you’d need a very large battery plus a backup generator. A grid-connected system earning export payments costs far less.
Do you need planning permission for a 4kW system?
Usually not. In England, rooftop panels on houses are generally permitted development, provided they don’t stick out more than 200mm from the roof slope. Listed buildings, some conservation areas, flats and ground-mounted arrays need checking with your council first.
Is a plug-in solar kit an alternative to 4kW on the roof?
Only if you can’t use your roof. Plug-in kits up to 800W became legal in Great Britain on 27 August 2026. They make roughly a fifth of a 4kWp roof system’s output, earn no export payments and are limited to one per household.
Can you add more panels to a 4kW system later?
Yes, but it costs more than fitting them at the start: scaffolding again, possibly a bigger inverter or G99 application, and new MCS paperwork. Some installers, including Octopus, don’t extend existing systems. See adding panels to an existing system.
How long does a 4kW installation take?
One to two days on the roof for most houses, plus scaffolding either side. From survey to switch-on, allow several weeks. See how long installation takes.
4kW is the right size for a typical gas-heated home, not a ceiling
A 4kW system costs £5,500-£7,500 installed, makes around 3,600 kWh a year in central Britain and pays back in roughly 8-14 years without a battery on September 2026 prices.
Two decisions change the result most. Pick a 12-18p export tariff over an open 3-6p one, and if your roof fits two or three more panels, price them while the scaffolding is up.
If an electric car or heat pump is on the way, treat 4kW as the minimum and get a 5-6kW quote alongside it before 0% VAT ends on 31 March 2027.
METHODOLOGY: Prices from DESNZ solar PV cost data 2025/26 (published 28 May 2026) and installers’ published prices, June-September 2026. Output figures are PVGIS-based planning estimates for unshaded south-facing roofs; the monthly split is an estimate for central England. Savings use Ofgem’s 26.32p/kWh electricity cap rate (1 October-31 December 2026, Direct Debit, no VAT) and export rates checked in summer 2026. The cost breakdown is illustrative.
DISCLAIMER: Figures are indicative and not financial advice. Real quotes vary with roof, region and access.
UPDATED: September 2026.
