What a 4kW solar system actually costs in 2026

A four kilowatt system is the size most UK households end up quoting for, and the price you are given for it will sit inside a wide band. The band is wide for reasons that have nothing to do with anyone overcharging you, and knowing what moves the number is more useful than knowing the average.

The short answer: the government’s own installation statistics put the median cost of small domestic solar at £1,780 per kilowatt in 2025/26, which prices a 4kW system at roughly £7,100 installed. Real quotes spread widely around that median, and most of the spread comes from three things: roof access, inverter choice, and how much of the job the installer subcontracts.

The official numbers, not brochure numbers

The Department for Energy Security and Net Zero publishes cost data for solar PV installations, built from the certification records of real installations rather than from surveys or advertised prices. The 2025/26 release puts the median cost of systems in the 0 to 4kW band at £1,780 per kilowatt, with the mean at £2,008 per kilowatt. The mean sits above the median because a minority of difficult, expensive jobs pull the average up, which is worth remembering when a comparison site quotes you “the average cost”.

Two things to note when applying those figures to your own quote:

• Multiply by the size on the quote, not the round number. A “4kW” system is often 3.8kWp or 4.3kWp once the actual panel count is fixed. At £1,780 per kilowatt, that difference is several hundred pounds of legitimate variation between two honest quotes.
• The per-kilowatt rate falls as systems grow. The same dataset puts the 4 to 10kW band’s median at £1,697 per kilowatt. Bigger systems spread the fixed costs, which is the strongest argument for not stopping at four kilowatts if your roof can take more.

VAT is the other thing the official figures make simple. The installation of solar panels in residential accommodation is zero rated for VAT until 31 March 2027, after which it reverts to the reduced rate of 5%, under HMRC’s energy-saving materials rules. A quote for a straightforward domestic supply-and-install should therefore carry no VAT at all today. If yours shows 20% on the qualifying work, query it.

Why four kilowatts became the default

Four kilowatts is roughly ten to twelve modern panels, which is about as much as fits comfortably on one pitch of a typical semi detached or terraced roof.

It also became the default partly through history. The old Feed-in Tariff paid its best rate below a 4kW threshold, so installers standardised on it. That scheme closed to new applicants in March 2019, but the habit outlived the incentive.

The modern reason systems cluster just under four kilowatts is grid paperwork. An inverter limited to 16 amps per phase, which is 3.68kW on a single phase supply, qualifies for the G98 “install then notify” process: the installer fits the system and tells your network operator within 28 days. Anything larger needs a G99 application approved before installation, which adds weeks and occasionally a network study. Many installers price and size to avoid that wait. It is worth asking whether yours is sizing to your roof or to their paperwork, because a larger array behind the same limited inverter, or a short G99 wait, is often better value over twenty five years than a system sized to the form.

Where the money goes

Split any quote into these buckets and the price stops looking arbitrary:

Panels. Ten to twelve modules. This is the part everyone focuses on and it is usually not the biggest line: hardware has fallen in price for a decade while scaffolding and labour have not, so on many jobs the panels themselves are now a minority of the total.

Inverter. One string inverter for a simple roof. A hybrid unit, which can run a battery later without being replaced, costs more up front and saves a second visit if storage is likely. Microinverters or optimisers cost more again and are worth it only if you have shading or panels on more than one plane.

Mounting. Rails, brackets and clamps, plus the labour to get them onto your particular roof. Slate takes longer than concrete tile and breaks more easily.

Scaffolding. Hired for the duration, and priced by the property rather than the system. A terraced house with rear access only is more expensive to scaffold than a detached house with a driveway, and the difference lands in your quote unchanged.

Electrical. Cable runs, isolators, and any consumer unit work. An older board sometimes needs replacing, which is a genuine extra rather than an upsell.

Certification and notification. MCS registration and the network operator paperwork. Small in cash terms and large in consequence: export tariffs under the Smart Export Guarantee require MCS or equivalent certification, so a cheap uncertified install usually forfeits export income entirely.

Margin and overhead. Sales, surveying, warranty provision and profit. This is the bucket that varies most between a national brand and a local firm, and the one a second quote exposes fastest.

A worked example: what £7,100 buys back

Take the median 4kW system at £7,121, which is the DESNZ 0 to 4kW median of £1,780.25 per kilowatt multiplied by four. Our calculator assumes 850kWh of annual generation per kilowatt as a UK average for a south facing roof, so call it 3,400kWh a year.

What that generation is worth depends on how much of it you use yourself. Electricity you consume as it is generated replaces imports that cost 26.11p per kWh on the current price cap, per Ofgem’s July to September 2026 cap rates. Electricity you export earns an export rate instead: the most widely held example is Octopus Outgoing at 12p per kWh.

Without a battery, a household that is out most weekdays typically self consumes somewhere near half of what it generates. Splitting 3,400kWh evenly as an illustration:

• 1,700kWh self consumed at 26.11p is about £444 a year
• 1,700kWh exported at 12p is about £204 a year
• Total: roughly £650 a year, in today’s money

Set against £7,121, that is a simple payback of about eleven years, with the panels warrantied well beyond it. Every one of those inputs moves the answer: a home worker self consuming more of the generation, a better export tariff, or a cheaper install all shorten it, and a north facing roof or a low usage household lengthens it. That is why we publish the working rather than a single national payback figure, and it is exactly what the calculator models with your own numbers instead of these illustrative ones.

The number that lets you compare quotes

Take each quote and divide the total by the system size in kilowatts peak. That gives cost per kWp, and it is the only figure that compares two quotes for differently sized systems. It is also the figure you can hold against the official medians above: £1,780 per kilowatt for small systems, £1,697 for the next band up.

Do it now for anything you are holding. A quote that looked expensive often turns out to be specifying more capacity; a cheap one is frequently smaller than the conversation implied. A genuine difference in cost per kWp is telling you something real about equipment, access or margin, and you can ask about it directly. Before comparing totals, also check what each quote excludes, which is a big enough subject that we keep a separate checklist of what belongs in a solar quote.

What makes your roof cheaper or dearer than average

• Access. Whether scaffolding can go up easily, and whether there is anywhere to park.
• Roof covering. Slate and clay are slower and more fragile than concrete.
• Pitch and height. Steeper and higher is slower.
• Cable run. The distance from the array to the consumer unit.
• Number of planes. Splitting an array across two orientations adds electronics.
• Existing electrics. An old consumer unit or no spare way.

None of these appear on a price list, and all of them are why a survey matters more than a phone quote. They are also why the official data shows a wide spread around its median: the spread is mostly roofs, not greed.

What to do with the figure

The cost only means something next to what the system saves, and the saving depends on your usage and your tariff rather than on the roof alone. A cheap installation with a poor return is still a poor investment, and a more expensive one that fits your consumption pattern can pay back sooner.

Get the cost per kWp for each quote, get the generation estimate and the tool that produced it, then put your own usage and your own import and export rates into the ROI calculator and see what falls out. That is a much stronger position than comparing two totals.