100kW Commercial Solar Cost UK 2026: £85-110k, 6yr Payback

A 100 kW commercial solar install in 2026 sits at one of the more interesting points on the cost curve. You move out of G98 simplicity and into G99 territory (which slows the process and demands proper DNO planning), but the per-kW economics improve, the panel count justifies a serious procurement effort, and 100% AIA still covers the entire project under the £1m annual cap. The numbers below are based on real PVSyst modelling against half-hourly meter data, not vendor brochure assumptions.

Who 100kW commercial solar fits

The 100 kW band lands cleanly across a specific group of UK businesses. Mid-format hotels (40–80 rooms) with year-round occupancy and steady daytime conference, kitchen and laundry load. Independent boarding and academy schools with extended-day operations, sports halls, kitchens and ICT load. Care homes and nursing homes above 40 beds where laundry, kitchen, cooling and lighting drive 24/7 daytime baseload. Light-industrial units of 1,000–2,500 sqm housing engineering, food processing, plastics or assembly. Mid-format DIY, garden centres, B&M-style retail and supermarkets up to 1,200 sqm sales floor. The common threads: roof footprint of 600+ sqm, three-phase electrical supply with good headroom, daytime baseload of 25 kW plus, and corporation tax exposure that lets AIA do its work.

What you actually get for £85,000–£110,000

2026 turnkey pricing for a 100 kW system runs £85,000–£110,000 plus VAT. The kit specification at this scale is more substantial than the 50 kW band: typically 185 tier-1 mono panels (Trina Vertex, JA Solar Deep Blue, Longi Hi-MO 6), commercial-grade three-phase string inverters (Sungrow SG-CX, Solis-S6, Huawei SUN2000-100KTL or SMA Tripower CORE) usually configured as 2 x 50 kW or 3 x 33 kW for redundancy, a galvanised-steel mounting system, an MCS-compliant DC and AC switchgear assembly, surge and arc-fault protection, a dedicated PV consumer unit, a half-hourly monitoring stack with sub-string data, full G99 connection paperwork (including Type Test certificates, witness testing arrangements with the DNO and ENA G99 forms), and a chartered structural engineer's sign-off. Scaffolding and access equipment are itemised. Asbestos work, three-phase headroom upgrades and structural reinforcement sit on a separate line.

Roof footprint, mounting and shading

Budget 600 sqm of usable roof area for a 100 kW install. South-facing pitched roofs are ideal but east-west splits work well on flat-roof commercial buildings — east-west arrays generate around 92% of an equivalent south-facing system but produce a flatter, longer daily curve that often matches business load profiles better. We model both layouts and present whichever delivers better IRR for your specific consumption pattern. Mounting at this scale is steel-framed ballasted on flat roofs (typically Renusol, Schletter or K2) or rail-mounted on trapezoidal sheets. Asbestos-cement industrial roofs are common on pre-2000 buildings and need licensed removal before install — we work with HSE-licensed contractors and frequently roll roofing and PV into a single project under Land Remediation Tax Relief, which can recover a further 50% of the asbestos element through corporation tax.

G99 — why 100kW changes the timeline

Above 50 kW per phase you fall under G99 (formally ENA Engineering Recommendation G99) which requires a full Connection Application to your DNO before commissioning. Standard DNO turnaround for the offer letter in 2026 runs 6–12 months, sometimes longer in constrained network areas (parts of London, the Midlands and the south-east). The application costs £350–£1,500 to file. Around 60% of 100 kW G99 applications get a standard offer with no reinforcement charge; the rest face contributions ranging from £2,000 to £40,000+ for transformer or upstream cable upgrades. Our process: file the G99 application within two weeks of contract signature, run design and procurement in parallel, and time delivery to site for the week the connection offer is accepted. If reinforcement comes back high we re-engineer to stay below the threshold (export limiting, sometimes downsizing to 99 kW) and re-file before any capex commits.

Worked example — 100kW for a Yorkshire hotel

Real-shape project: a 60-room independent hotel in West Yorkshire, year-round occupancy averaging 72%, on-site restaurant and conference rooms, three-phase 400 A supply with 50 kVA headroom, 720 sqm flat membrane roof in serviceable condition, half-hourly meter data showing 380,000 kWh annual consumption with daytime baseload of 28 kW. Quoted £96,500 plus VAT for a 99.6 kW system (185 x 540 Wp JA Solar panels, 2 x Sungrow SG50CX inverters, K2 ballasted east-west mounting at 10 degrees). Modelled year-one yield 91,200 kWh. Self-consumption modelled at 74% (282 kWh per day on average, well-matched to baseload), so 67,500 kWh avoid the grid at 25p/kWh blended (£16,875 saved) and 23,700 kWh export at 6p/kWh SEG (£1,422). Total year-one benefit £18,300. AIA tax relief year one £24,125 against corporation tax. Simple payback 5.9 years, 25-year IRR 16.1%, 25-year NPV at 7% discount £382,400. The full PVSyst, financial DCF and meter analysis ship with every proposal.

Finance — where 100kW becomes interesting

At 100 kW scale, financing options open up meaningfully. Cash with AIA delivers the strongest IRR for any limited company with corporation tax exposure — a £95,000 install nets to £71,250 after the £23,750 AIA tax relief. Asset finance over 5–7 years (around £1,500–£1,800 per month over six years on £95,000 capex) is the route most of our 100 kW customers actually choose: the monthly finance payment is consistently lower than the monthly bill saving, so the project is cash-flow positive from month one and you preserve working capital for trading. 100 kW is the smallest scale at which PPA economics start to make sense — a 15-year fixed-rate PPA at 12–15% below current grid retail with end-of-term ownership transfer is realistic, particularly for businesses with limited corporation tax position. We model all three routes against your specific accounts on every finance options page assessment.

Where 100kW lands hardest by sub-vertical

From our 2025 install book the 100 kW band overweighted heavily across five sub-verticals: mid-size hotels and conference venues (year-round daytime demand, kitchen and laundry load), independent and academy schools (extended-day operations, ICT load, kitchens), care homes above 40 beds (24/7 demand including laundry and cooling), light-industrial units 1,000–2,500 sqm (single-shift manufacturing, food processing, packaging), and mid-format retail and supermarkets (chiller load, lighting, tills). Common threads: 600+ sqm roof, three-phase 200 A+ supply, daytime baseload comfortably above 20 kW, and a financial position where £25k of year-one tax relief makes the AIA economics decisive.

The 100kW survey process

Survey runs in two passes, same as smaller installs but with a heavier feasibility step. Desk feasibility uses your half-hourly DCP228 export, satellite and Lidar roof modelling, draft G99 grid feasibility against the local DNO's heat map, and a full PVSyst yield run — turnaround five working days. If the desk feasibility shows worthwhile IRR (we don't take projects forward where 25-year IRR falls below 10%), we visit on-site for structural survey, asbestos register check, electrical infrastructure assessment and roof condition inspection. Final fixed-price proposal lands within seven working days of the site visit, with the full meter analysis, yield model, financial DCF and grid feasibility narrative attached.

Inverter strategy at 100kW — string versus central

Inverter selection at 100 kW is more nuanced than at smaller scales. We typically specify two or three commercial-grade three-phase string inverters rather than a single central inverter for three reasons. First, redundancy: a single 100 kW inverter failing takes the entire system offline; two 50 kW or three 33 kW inverters mean a single failure costs you only one-third of generation while the failed unit is replaced. Second, MPPT granularity: split arrays facing east-west, or arrays with awkward shading, benefit from multiple maximum power point trackers — five MPPTs across three inverters often delivers 3–5% more annual yield than two MPPTs across a single central unit. Third, lead times: 50 kW inverters from Sungrow, Solis and SMA typically have stock availability of 4–8 weeks in 2026, whereas larger central units routinely run 16–20 weeks lead time, slowing project delivery. We model both options and present the IRR comparison in the proposal — sometimes a single central inverter is the right answer, but usually a multi-inverter string topology wins on availability and yield.

Cabling, switchgear and electrical infrastructure

The electrical work on a 100 kW project is more substantial than most owners expect. DC cabling runs from each panel string through MC4 connectors, into combiner boxes, to the inverter — typically 80–120 metres of DC cable per string with surge protection devices and arc-fault detection on every string. AC cabling from inverter output to the customer's switchgear typically runs 30–80 metres depending on inverter location, sized to accommodate the full inverter output current with adequate voltage drop margin (we target sub-2% voltage drop end-to-end). At the point of connection we install a dedicated PV consumer unit with main isolator, AC isolator, kWh meter for SEG export reporting, and protective devices. If your existing main switchgear has limited spare way, we add a tap-off enclosure — typically £1,500–£4,000 of switchgear work itemised on the quote. Earthing and bonding is upgraded to suit the additional DC array, with TT or TN-C-S earthing systems handled per BS 7671. SPD (Surge Protection Device) coordination across DC, AC and signal circuits is standard — protects the inverter and the wider building electrical system against lightning-induced transients. None of this is exotic; it's just thorough.