100kW commercial solar — cost, ROI & payback

A 100 kWp system sits at the point where commercial solar economics get attractive: it is large enough that fixed costs (scaffolding, design, DNO application, commissioning) spread across a lot of capacity, but small enough to fit on a single large roof. In 2026, turnkey supply-and-install pricing from MCS-certified installers is typically £850 to £1,050 per kWp at this scale, so a 100 kWp system usually lands at around £85,000 to £105,000 (ex-VAT).¹

As a sanity check, per-kWp cost falls steadily with size — a 10 kW system is often £1,100 to £1,300 per kWp, while a 250 kW system can reach £650 to £750 per kWp. A 100 kW system sits comfortably in between. You can compare the smaller and larger price points on our size pages: 50kW, 100kW and 250kW.

Every roof is different. Trapezoidal metal roofs are cheaper to mount than standing-seam or older asbestos-cement; a ground-mount or car-port structure costs more than a simple roof fix. The only way to a firm number is a site-specific quote from one installer who has seen your roof, half-hourly data and grid connection.

The table below shows an illustrative apportionment of a mid-range £90,000 (ex-VAT) 100 kWp installation. The split varies by roof type and installer, but it shows where the money goes.

At 100 kW the grid connection usually needs a G99 application to the Distribution Network Operator rather than the simpler G98 used for small systems — this is normal at this scale and the installer handles it, but it does add time and cost, which is why we show it as a line item.

UK commercial rooftops typically yield around 950 kWh per kWp per year, so a 100 kWp system generates roughly 95,000 kWh a year (an estimate — actual output depends on your latitude, roof pitch, orientation and shading). At about 7 m² per kWp, you need around 700 m² of usable, unshaded roof.

The single biggest driver of return is self-consumption — the share of generation you use on site rather than export. Power you use on site displaces electricity you would otherwise buy at ~25p/kWh; power you export earns only a few pence via a Smart Export Guarantee tariff.^1,5 Commercial self-consumption is typically 60% to 85% depending on sector: a two-shift factory or a cold store that runs all day self-consumes far more than a Monday-to-Friday office.

For a sector-specific view, see warehouse, factory, farm, retail and hotel.

The worked example below assumes 70% self-consumption, a 25p/kWh commercial import price and a 5p/kWh export rate. These are illustrative — your import tariff, in particular, has a large effect, so use your own contract rate.^1,5

Over a 25-year panel life, panel output degrades slowly (roughly 0.5% a year) while grid electricity prices tend to rise — these broadly offset, so holding the annual benefit flat at ~£18,000 is a reasonable simplification. On that basis the lifetime net benefit is an estimated £350,000-plus after the upfront cost, before any tax relief. Inverters usually need replacing once in that period, which we would expect a good quote to flag.

Whether to buy outright or take a Power Purchase Agreement (no upfront cost, you buy the power) changes these numbers materially — see buy vs PPA.

Two tax points matter for a 100 kW commercial system, and both are worth checking with your accountant.

Capital allowances. Solar PV is treated by HMRC as special-rate plant (assets with a long expected life). Special-rate plant does not qualify for 100% full expensing, which is reserved for main-rate assets. Instead, incorporated businesses can claim a 50% first-year allowance on qualifying special-rate spend, deducting 50% of the cost in year one, with the balance written down at 6% a year in the special-rate pool thereafter.^2,3 Many businesses can alternatively cover the spend under the Annual Investment Allowance (100% up to £1 million), if AIA is available — your accountant will confirm the most efficient route for your accounts.

VAT. The 0% VAT rate applies to domestic solar only. Commercial installs are charged the standard 20% VAT, which a VAT-registered business normally reclaims through its VAT return.⁴ That is why the costs on this page are shown ex-VAT.

A 100 kWp system is a strong fit where three things line up:

• Roof area: around 700 m² of usable, structurally sound, largely unshaded roof — a mid-to-large warehouse, factory unit, agricultural shed, distribution centre or retail park unit.
• Electricity spend: a business with an annual electricity bill in the region of £40,000 to £120,000-plus, so that a large share of ~95,000 kWh is used on site rather than exported.
• Daytime load: operations that run in daylight hours — production lines, refrigeration, EV charging, pumps, HVAC — which push self-consumption toward the higher end.

If your roof is smaller or your bill lower, a 50kW system may fit better; if you have a very large roof and a big daytime load, look at 250kW. The commercial solar hub and our how to choose guide walk through sizing against your own half-hourly data.

The numbers on this page are anchors, not a quote. The variables that decide your actual return — roof condition, orientation, shading, your half-hourly load profile, your import tariff and your grid connection — are specific to your building and can only be assessed on site.

PlutoHome is a neutral introducer. We are not an installer and we do not install, finance or own systems. We match your enquiry with one MCS-certified commercial installer who quotes for your specific premises — never five firms all chasing the same lead.⁶

Request your single matched quote, or read buy vs PPA first if you are weighing up how to fund it.