How the Business Solar Calculator Works

Step 1 — Establishing your annual consumption

The calculator needs to know how much electricity your business uses across a year. If you enter annual kWh directly from your bill, it uses that figure unchanged — it is the most accurate input you can give. If you only know your monthly spend, it converts at your import rate:

Annual kWh = (monthly spend × 12) ÷ import rate

A business spending £2,500 a month at 26p/kWh is consuming roughly 115,000 kWh a year. The conversion deliberately ignores standing charges, which typically make up 3–8% of a commercial bill; ignoring them slightly overstates consumption and therefore slightly oversizes the recommendation, which the roof-area cap then corrects in most real cases. If your bill shows your actual unit rate, enter it — the conversion tightens immediately.

Step 2 — Recommending a system size

To size from a bill, an annual kWh figure or a roof area in one place, use the commercial solar system size calculator; it applies the same regional yields and the same 6.5 m² per kWp rule, and adds a daytime-load rule this page does not.

Three inputs can set the size, in a strict order of precedence. If you enter a desired system size in kWp, the calculator uses it without argument — you may be matching an installer's quote or testing a scenario. Otherwise it computes a consumption-matched size:

Size (kWp) = annual kWh ÷ regional yield (kWh per kWp)

This sizes the system so annual generation roughly equals annual consumption — the upper bound of what usually makes economic sense, since generation beyond consumption is all exported at the lower SEG rate. If you also enter a usable roof area, the recommendation is capped at what physically fits:

Maximum size (kWp) = roof area (m²) ÷ 6.5

The 6.5 m² per kWp density reflects 2026 commercial panels at around 440–460W in a ~2.1 m² frame, plus row spacing, walkways, and clearance from roof edges and plant. Flat roofs with ballasted east-west arrays can do slightly better; heavily serviced roofs with skylights and HVAC do worse. If you know your layout, enter the kWp directly.

Step 3 — Annual generation by region

Generation is system size multiplied by your regional yield. The calculator uses five UK regional figures for a well-oriented, largely unshaded commercial rooftop: North England 850, Midlands 950, South England 1,050, Scotland 800 and Wales 950 kWh per kWp per year. These are planning values for UK rooftops rather than measured output from named arrays; the regional yields page covers the spread within each region and what orientation and pitch do to the number.

Step 4 — Valuing the electricity

Each generated kWh ends up in one of two places. Used on-site, it displaces a unit you would have bought at your import rate — defaulted to 26p/kWh, a mid-table 2026 fixed-contract rate for UK businesses (the electricity prices page shows the full range by business size). Exported, it earns your Smart Export Guarantee rate in Great Britain (in Northern Ireland, which has no SEG, your supplier’s export tariff) — defaulted to 12p/kWh.

The split between the two is your on-site use percentage, defaulted to 65% — typical for a business operating five or six days a week with normal daytime load. The calculator applies one safeguard: on-site use can never exceed your actual annual consumption. If your inputs imply a system generating far more than you use, the surplus is automatically valued at the export rate instead, which is exactly what happens in reality.

Year-1 value = (on-site kWh × import rate) + (exported kWh × export rate)

Step 5 — Costing the system

The capex estimate uses 2026 UK fully-installed bands: ~£900–£1,100 per kWp below 50kWp, and ~£750–£900 per kWp from 50kWp to 250kWp. "Fully installed" means panels, inverters, mounting, DC and AC electrical work, scaffolding or access, the G99 grid application, structural sign-off and commissioning. Above 250kWp the calculator holds the 50–250 band and marks the result as indicative — at that scale, DNO engineering and procurement strategy move the price more than any rate card can capture.

Payback and the 25-year figure use the midpoint of the band. Where your project actually lands depends mostly on roof type (trapezoidal steel is cheapest; fragile or membrane roofs cost more), access, and whether your grid connection needs reinforcement.

Step 6 — Payback and the 25-year figure

For the same system’s cash, discounted and after-tax payback, with running costs and the inverter included, use the commercial solar payback calculator.

Simple payback = mid-band capex ÷ year-1 value. No financing cost, no tax relief, no electricity price inflation, no maintenance deduction. Those omissions partly cancel: maintenance and insurance (typically 1–2% of capex a year) lengthen real payback, while rising power prices shorten it. The Annual Investment Allowance relieves the cost in year one, but the saving is taxed too, so it moves payback in years very little.

The 25-year indicative benefit degrades output by 0.5% a year (tier-1 panel warranties typically guarantee 84–87% of original output at year 25), holds prices flat, sums the 25 years of value, and subtracts the mid-band capex. Holding prices flat for 25 years is the single most conservative assumption in the tool — every above-zero rise in electricity prices makes the real outcome better than the number shown.

What the tool deliberately leaves out

Batteries, EV charging, export limitation, three-phase upgrade costs, and power factor correction are all real considerations the calculator does not model — each one is site-specific enough that a generic assumption would mislead more than it informs. The same goes for your half-hourly load profile, which is why the on-site use percentage is an input rather than an output. When you want those modelled properly, the quote form sends us your details and we reply by email; an installer you choose can model your actual meter data. For fully worked numbers using this exact method, see the worked examples. To run the same method over 25 years with tax and a discount rate, use the commercial solar ROI calculator.