50kW vs 100kW Solar System Cost UK: Factory Guide

50kW vs 100kW solar for a UK factory: which size actually pays?

Two systems dominate quotes for mid-sized UK manufacturing sites: the 50kW array and the 100kW array. Both are firmly in commercial territory, both need a full G99 grid application, and both are strong candidates for capital allowances. The right choice comes down to three things — how much roof you have, how much power you use during daylight hours, and how quickly you want your money back.

This guide runs the worked numbers side by side using current mid-2026 UK figures, so you can see exactly where each system fits. If you want the full pricing picture first, our factory solar panel costs breakdown covers every array size from 50kW to 1MW.

The headline comparison

The pattern is clear: doubling the array size roughly doubles the cost, the generation and the savings, but the payback window stays broadly the same. The 100kW system tends to edge slightly cheaper per kWp because installation, scaffolding and G99 engineering costs are spread across more panels.

Cost: what you actually pay

UK commercial solar sits at roughly £700–£1,050 per kWp installed (ex-VAT), and that rate falls as the system scales. A 50kW factory array typically lands at £40,000–£52,000, while a 100kW array runs £75,000–£105,000. Remember that commercial solar carries the standard 20% VAT — the 0% rate is residential-only and runs to 31 March 2027 — but if your business is VAT-registered, that VAT is recoverable, so it is a cash-flow item rather than a true cost.

Where you land inside those bands depends on roof type (pitched steel vs flat membrane with ballast), inverter choice, cable runs and whether you add battery storage. For a full itemised view of every array size and what drives the price, see our commercial solar panel cost guide, and if you are weighing solar as a wider capital project read our overview of business solar panels.

Generation and savings: the daylight-load test

A UK array generates roughly 900–1,000 kWh per kWp each year. So a 50kW system produces about 45–50 MWh annually, and a 100kW system about 90–100 MWh. At commercial electricity rates of around 28–32p/kWh, every unit you use on site instead of importing is worth real money.

The critical variable is self-consumption. Factories that run machinery, compressors, extraction or lighting through the working day soak up most of what the panels make, so nearly all the generation offsets 30p grid power. A 100kW array on a high-daytime-load site can displace £18,000–£25,000 of grid spend a year. A 50kW array on the same profile saves roughly £10,000–£13,000.

If your daytime demand is modest, a 100kW system may push surplus power to export — paid at 1–12p/kWh under the Smart Export Guarantee (best-in-market around 12p), or better via a negotiated PPA on larger sites. Export is worth roughly half to a third of what self-consumption saves, so oversizing beyond your daytime load dilutes the return. This is exactly the sizing judgement a good installer makes for you.

Continuous-load sites tip the maths

Sites with round-the-clock demand — chilled warehouses, process plants and solar for cold storage operations — have the flattest, highest daytime baseload of any commercial building. For those, the 100kW system almost always wins, because there is enough on-site demand to self-consume nearly everything it generates.

Roof area: the hard constraint

Sizing often comes down to how much usable roof you have. As a rule of thumb, budget around 5–8 m² of roof per kWp once you allow for tilt, walkways, rooflights, plant and set-backs. That puts a 50kW array at roughly 250–400 m² and a 100kW array at 500–800 m² — about 100–110 versus 200–220 modern panels.

Flat roofs need more space than pitched ones because ballasted frames are spaced to avoid inter-row shading. A structural check matters too: older steel-frame factory roofs sometimes need reinforcement to carry a 100kW array plus ballast, which can nudge you back toward 50kW. A site survey by qualified commercial solar installers settles this quickly.

Payback and tax relief

Both systems typically hit simple payback in 4–6 years and 3.5–5 years once capital allowances are factored in. Since the Autumn Statement 2023, the Annual Investment Allowance (AIA) gives 100% first-year tax relief on qualifying plant up to £1m a year — and it covers solar. At 25% corporation tax, that means a £42,000 50kW system yields around £10,500 of year-one relief, and an £85,000 100kW system around £21,000. Both sit comfortably under the £1m AIA ceiling, so the full first-year deduction applies.

If your total qualifying spend in the year exceeds £1m, solar is a special-rate asset, so the balance above the AIA gets the permanent 50% First-Year Allowance, then 6% writing-down allowance thereafter. Note that commercial solar does not qualify for full expensing — that relief is for main-rate plant only. For the incentives picture in full, see our guide to commercial solar incentives, and be realistic: there is no broad UK cash grant for commercial solar, so the economics rest on tax relief plus self-consumption, not a handout.

Which size fits which factory?

Choose 50kW if…

• Your annual electricity bill is roughly £15,000–£30,000
• Usable roof is limited (under ~400 m²) or the structure is older
• You want a smaller capital outlay to test solar before scaling
• Daytime load is moderate rather than continuous

Choose 100kW if…

• Your bill exceeds ~£35,000–£40,000 a year
• You have 500 m²+ of sound, unshaded roof
• You run machinery, refrigeration or a heavy daytime baseload
• You want the best cost-per-kWp and the largest absolute saving

Many factories phase it: install 50kW now on the strongest roof section, then extend to 100kW when the first system has proven itself. Because the G99 application defines your export capacity, it is worth telling your DNO the eventual target size up front to avoid a second application later.

Getting a like-for-like comparison

The quickest way to know whether 50kW or 100kW is right for your site is a proper roof survey and a half-hourly load analysis — not a rule of thumb. Because we are independent and supplier-neutral, we can compare quotes from vetted UK installers on identical terms, so you see genuine like-for-like pricing on both system sizes rather than one company's pitch.

Explore the best commercial solar companies we benchmark against, learn how commercial solar installation runs from survey to switch-on, or go straight to the numbers for your building. Whatever the size, solar panels for factories only pay when they are sized to your real daytime load — and that is exactly what an independent assessment gives you.

Use our free quote comparison to get matched with vetted commercial solar panel installers and receive side-by-side proposals for both a 50kW and a 100kW array — no obligation, no sales calls.