20kW commercial solar — cost, ROI & payback

At the 20kW scale, UK installed prices in 2026 sit at roughly £1,000 to £1,200 per kWp (ex-VAT) for a straightforward rooftop job, which puts a full system around £21,000, or between about £19,000 and £24,000.¹ Per-kWp cost falls as systems get bigger, so a 20kW install is more expensive per unit than a 100kW+ scheme but cheaper than a small 10kW one. The figures below are an estimate for a typical single-phase or small three-phase rooftop site; a survey will confirm yours.

A well-sited UK system yields roughly 950 kWh per kWp per year, so 20kWp produces about 19,000 kWh annually — enough to make a real dent in a mid-sized commercial bill.² Output depends on orientation, pitch and shading; a survey uses your postcode and roof to model it precisely.

• Roof area needed: about 7 m² per kWp, so roughly 140 m² for 20kW.
• Annual generation: ~19,000 kWh (an estimate for a typical site).²
• Panel count: around 36–40 modern panels at ~500–550 W each.
• Degradation: modern panels lose roughly 0.5% output a year, so a 25-year system still generates around 88% of its year-one figure in its final year.⁶

The saving comes from two things: the grid electricity you no longer buy (worth most), and a small payment for surplus you export. A typical daytime commercial site — offices, workshops, retail, light industrial — self-consumes around 60–85% of what it generates; we model 70% here. We value avoided grid power at about 26p/kWh, in line with average non-domestic prices,³ and exported units at a 15p/kWh Smart Export Guarantee rate.⁴

On a £21,000 outlay and a ~£4,300 annual saving, simple payback is around 5 years before tax relief. Because the saving is dominated by avoided grid cost, a higher electricity tariff or higher self-consumption shortens payback; a lower one lengthens it.

Over a 25-year life, if prices simply held flat the system would save on the order of £95,000–£100,000 — an illustration, not a promise, since real electricity prices and your usage will differ. Allowing for panel degradation and one likely inverter replacement (~£2,500), that points to an estimated lifetime net benefit near £75,000 after the original cost. Compare ownership against a no-upfront-cost route on our buy vs PPA page.

Two tax points materially affect the real cost — confirm both with your own accountant.

• VAT: the 0% VAT rate applies to domestic solar only. Commercial solar pays 20% VAT, but a VAT-registered business normally reclaims it, so the net cost usually returns to the ex-VAT figure.⁵
• Capital allowances: solar PV is classed as special-rate plant. That means it does not qualify for 100% full expensing (which is for main-rate plant), but special-rate assets qualify for the 50% first-year allowance, made permanent from April 2023.⁶ Separately, most businesses can instead use the Annual Investment Allowance (100% up to £1m a year), which comfortably covers a £21,000 system — so many buyers write the full cost off in year one.⁷

A 20kW system suits businesses that use meaningful power through the working day and have around 140 m² of usable roof. As a rough guide, it fits sites with an annual electricity bill of roughly £5,000–£12,000 and steady daytime demand, where most generation is used on-site rather than exported.

• Small warehouses and trade counters — see warehouse solar.
• Workshops and light manufacturing — see factory solar.
• Farm buildings and barns — see farm solar.
• Shops, showrooms and hospitality — see retail and hotel solar.
• Schools and community buildings — see school solar.

Larger sites should compare against 30kW, 50kW and 100kW systems, where per-kWp cost is lower. Smaller loads may fit a 10kW system. The full range sits on our commercial solar hub.

The numbers on this page are sanity anchors. The only figures that matter for your building come from a site survey by an MCS-certified commercial installer who checks your roof structure, shading, half-hourly load and DNO connection.⁸

PlutoHome matches you with one vetted MCS installer for your area and system size — not five firms competing to call you. We are a neutral introducer: we do not install, finance or own systems, and we take no view on brand or kit.

Before you commit, read how to choose a commercial installer and weigh ownership against a no-upfront route on buy vs PPA. When you are ready, request an introduction and we will match you to one installer for a proper survey and quote.