Worked Examples: Business Solar at Three Sizes

Example 1 — 30kWp on a South England office

Illustrative scenario, not a real customer. Take a professional services firm near Reading that uses 85,000 kWh a year (about £1,840 a month at its 26p contract rate). Its flat roof has around 220 m² of clear space — enough for 33kWp at 6.5 m² per kWp — and the firm settles on a 30kWp east-west array. Offices have decent but not exceptional daytime load, so the default 65% on-site use is fair.

• Generation: 30 kWp × 1,050 kWh/kWp (South England) = 31,500 kWh/year
• Used on-site: 31,500 × 65% = 20,475 kWh → × 26p = £5,324
• Exported: 11,025 kWh → × 12p SEG = £1,323
• Year-1 value: £5,324 + £1,323 = £6,647
• Capex (under 50kWp band, £900–£1,100/kWp): £27,000–£33,000, midpoint £30,000
• Simple payback: £30,000 ÷ £6,647 = 4.5 years
• 25-year net benefit: £6,647 × 23.56 (0.5%/yr degradation) − £30,000 ≈ £126,600

Worth noticing: the system only covers about a quarter of the firm's consumption. Sizing to the full 85,000 kWh would need 81kWp — more than twice what the roof holds. The roof, not the bill, is the binding constraint on most offices, which is exactly why the calculator caps its recommendation at roof capacity.

Example 2 — 100kWp on a Midlands warehouse

Illustrative scenario, not a real customer. Take a distribution business outside Leicester that uses 180,000 kWh a year across conveyors, charging, lighting and offices. Its steel portal-frame roof could take three times this system, so the array is sized toward consumption instead: 100kWp, the most common commercial install size in the UK. Six-day operation with steady daytime load supports the default 65% on-site use.

• Generation: 100 kWp × 950 kWh/kWp (Midlands) = 95,000 kWh/year
• Used on-site: 95,000 × 65% = 61,750 kWh → × 26p = £16,055
• Exported: 33,250 kWh → × 12p SEG = £3,990
• Year-1 value: £16,055 + £3,990 = £20,045
• Capex (50–250kWp band, £750–£900/kWp): £75,000–£90,000, midpoint £82,500
• Simple payback: £82,500 ÷ £20,045 = 4.1 years
• 25-year net benefit: £20,045 × 23.56 − £82,500 ≈ £389,700

The step down from the sub-50kWp cost band to £750–£900/kWp is why warehouses are the sweet spot of UK commercial solar: scale economies arrive just as roof space stops being a constraint. Note also that the system covers a little over half of consumption (61,750 of 180,000 kWh) — there is headroom to go bigger if the business electrifies its forklift fleet or adds cold storage later.

Example 3 — 250kWp on a North England factory

Illustrative scenario, not a real customer. Take a precision engineering plant near Sheffield that runs two shifts and 600,000 kWh a year. Machine load through daylight hours is heavy and continuous, so on-site use is set at 80% rather than the default — exactly the kind of adjustment the calculator's editable assumptions exist for. The array fills most of a 1,700 m² north-light roof's usable south faces: 250kWp.

• Generation: 250 kWp × 850 kWh/kWp (North England) = 212,500 kWh/year
• Used on-site: 212,500 × 80% = 170,000 kWh → × 26p = £44,200
• Exported: 42,500 kWh → × 12p SEG = £5,100
• Year-1 value: £44,200 + £5,100 = £49,300
• Capex (top of the 50–250kWp band, £750–£900/kWp): £187,500–£225,000, midpoint £206,250
• Simple payback: £206,250 ÷ £49,300 = 4.2 years
• 25-year net benefit: £49,300 × 23.56 − £206,250 ≈ £955,000

Despite the lowest regional yield of the three examples, the factory posts the strongest absolute return — its 80% on-site use converts most generation at 26p rather than 12p. Load profile beats geography. At this scale the G99 grid application and possible DNO reinforcement also become real cost variables, which is why the calculator flags anything above 250kWp as indicative.

What the three examples have in common

All three land in the 4–5 year payback range before tax relief. The Annual Investment Allowance deducts the full capex from taxable profits in year one, which returns 19–25% of the cost as tax relief, but the saving is taxed as well, so payback in years moves far less than that (the payback calculator shows the after-tax figures on your own inputs). None of them assume electricity price inflation; every year prices rise, real payback shortens further. And in all three cases the single number most worth interrogating is the on-site use percentage — check it against your operating hours before trusting any output, ours included. When the payback earns a real quote, take these figures to an MCS-certified installer, who can re-run them against your half-hourly data and a roof survey. To put a sized example through a 25-year cash flow with tax, use the commercial solar ROI calculator. To check the sizes themselves against your own bill and roof, use the commercial solar system size calculator, and for simple, discounted and after-tax payback on any of them, the commercial solar payback calculator. The methodology page documents each formula used above, and the 2026 price data justifies the 26p default.