A 250kW rooftop array is where commercial solar stops being an add-on and starts being infrastructure. It’s the size you see on a mid-to-large distribution shed, a multi-building manufacturing site, a big agricultural holding or a supermarket. At this scale the economics get better per kilowatt, but the engineering — grid connection, structural loading, half-hourly metering — gets more demanding. If you’ve been quoted somewhere between £180,000 and £280,000 and want to know what’s reasonable, here is the real cost, output and payback maths for 2026.
What a 250kW system actually costs installed
Commercial solar is priced per kWp (kilowatt-peak). At 250kW, expect £740–£1,060 per kWp in 2026, which puts a full installation at roughly £185,000–£265,000. That’s a lower per-kWp figure than a 50kW or 100kW system, because the fixed costs — scaffolding or MEWP access, the DNO application, design and project management — are spread across far more capacity.
The price includes the mounting system, string or central inverters, DC and AC cabling, the G99 grid-connection application, commissioning and handover. What moves you within that range:
- Roof type and condition — a modern standing-seam or trapezoidal metal roof is quick and cheap to fix to; an ageing asbestos-cement or ballasted flat roof adds cost.
- Grid connection — a G99 application onto a substation with spare capacity is straightforward; one needing DNO reinforcement can add weeks and tens of thousands of pounds.
- Structural loading — a 250kW array is a meaningful dead and wind load; a structural survey sometimes flags strengthening work, especially on older portal frames.
- Inverter strategy — central inverters can be cheaper at this scale, while distributed string inverters add resilience and simpler fault isolation.
How much a 250kW system generates
At UK yields of roughly 850–950 kWh per kWp, a 250kW system produces around 210,000–240,000 kWh a year. On commercial electricity at 22–27p/kWh, generation you use on-site is worth £46,000–£65,000 a year in avoided import — before you count any export income.
The single biggest lever on the financial case is self-consumption: the share of that generation you use on-site rather than export. A 24/7 operation like cold storage or manufacturing might self-consume 80%+ of a 250kW array; a nine-to-five office will export more midday surplus at lower Smart Export Guarantee rates. Sizing the system to your load profile — not just your roof — is what separates a four-year payback from a six-year one.
Payback and the tax picture
Most 250kW commercial installs land on a 4–6 year pre-tax-relief payback. Add a battery to shift midday surplus into evening peak demand and you improve both self-consumption and demand-charge savings, typically pulling payback toward the shorter end. Solar PV can also qualify for the Annual Investment Allowance, letting a limited company offset the cost against taxable profits and lifting the after-tax return — a point worth modelling with your accountant before you sign.
For a like-for-like view of smaller systems, see our breakdowns of 100kW solar system cost and 50kW solar system cost; the per-kWp figure falls as you scale up.
Getting an accurate 250kW quote
At this size, a credible quote should always include a structural assessment, a G99 grid-connection plan and a self-consumption model built from your actual half-hourly consumption data — not a generic template. Any quote that skips those is guessing. Because a 250kW project is a serious capital decision, it pays to work with an installer that specialises in commercial-scale systems and understands the DNO process in your region, rather than a domestic firm scaling up.
The right size for your site is ultimately set by your roof area, your electricity demand and your grid headroom — so treat these figures as a planning baseline, then get a site-specific survey to firm up the numbers.