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

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 sit in the mid-range of published UK irradiance data and real metered output from commercial 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 — 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

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 the Annual Investment Allowance and rising power prices shorten it — usually by more.

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 starts a desk feasibility from your actual meter data. For fully worked numbers using this exact method, see the worked examples.