Business Solar Calculator — Real UK Numbers in 60 Seconds

What this business solar calculator actually does

Most solar calculators are lead forms in disguise: three sliders, a hidden formula, and a "request your results" button that swaps your email address for a number you cannot check. This one is different by design. Every input is visible, every assumption is editable, and the full method is documented on the how it works page — down to the regional yield figures and the cost-per-kWp bands behind the capex estimate.

The tool answers the four questions every UK business asks before talking to an installer. What size system fits my consumption and my roof? What will it cost to install in 2026? What is it worth in year one? And how long until it pays for itself? You can answer all four from a single monthly bill figure, although the more you can tell it — annual kWh from your bill, usable roof area in square metres, your actual unit rate — the tighter the output gets.

It is built for businesses, not homes. The cost bands start at small-commercial scale (around 15–30kWp, the size of a typical workshop or office roof) and run through the 50–250kWp range that covers most UK warehouses, factories and depots. Domestic systems are priced differently and a household calculator will serve you better there.

The numbers behind the tool, in plain English

Three sets of assumptions, all published on this site, drive the output. First, regional yield: a kWp of well-oriented rooftop solar generates about 1,050 kWh a year in South England, 950 in the Midlands and Wales, 850 in North England and 800 in Scotland. The spread matters — the same 100kWp system earns roughly £5,300 a year more in Kent than in Aberdeen at default rates. The regional yields page sets out where these figures come from and what moves them.

Second, 2026 installed cost. Below 50kWp, UK commercial installations currently price at roughly £900–£1,100 per kWp fully installed; from 50kWp to 250kWp, scale economies bring that down to £750–£900. A 30kWp office system therefore lands around £27,000–£33,000, while a 100kWp warehouse array comes in near £75,000–£90,000. These are full-scope figures — panels, inverters, mounting, scaffolding, the G99 grid application and commissioning — not bare hardware prices.

Third, the value of a solar kWh. Electricity you generate and use on-site displaces grid units at your import rate — defaulted to 26p/kWh, the mid-range of 2026 business contracts analysed on our commercial electricity prices page. Electricity you export earns a Smart Export Guarantee rate, defaulted to 12p/kWh. That gap is the engine of the whole calculation: a unit used on-site is worth more than twice a unit exported, which is why the on-site use percentage moves payback more than any other input.

How to read your results

The recommended system size is sized to your annual consumption — a system that generates about as much as you use across the year — and capped at what your roof can physically hold at 6.5 m² per kWp. This is the sweet spot for most businesses: big enough to matter, small enough that most generation gets used rather than exported. The capex band is a 2026 fully-installed range; where your project lands within it depends mostly on roof type, access and grid connection, which only a survey can settle.

Year-1 savings combines avoided import costs and export income. Simple payback divides mid-band capex by year-1 savings — no financing costs, no tax relief, no electricity price inflation. That makes it cautious on prices: every above-inflation rise in power prices shortens real payback. Tax works both ways: the Annual Investment Allowance relieves the cost in year one, but the saving is taxed as well, so after-tax payback in years comes out close to the pre-tax figure. The 25-year benefit applies 0.5% annual panel degradation and flat prices, then subtracts the capex — a deliberately cautious floor, not a sales projection.

If you want to sanity-check the tool against fully worked numbers, the worked examples page walks an office 30kW, a warehouse 100kW and a factory 250kW system through the identical method line by line.

What the calculator cannot tell you

Honest limits, stated plainly. It cannot see your half-hourly load profile, so the on-site use figure is an estimate you should refine — a cold store running refrigeration around the clock might hit 90%+, while a 9-to-5 office without weekend load might manage 50%. It cannot assess roof condition, shading from adjacent buildings, or whether your local DNO will accept the export connection without reinforcement. And it prices systems above 250kWp at the 50–250kWp band, flagged as indicative, because projects at that scale are genuinely priced per site.

None of those gaps change the arithmetic; they change the inputs. When you are ready for numbers somebody will put their name to, an installer you choose can turn your calculator result into a fixed-price proposal built from your actual meter data. If you would like to send us the result first, use the quote form and we reply by email.

From calculator to commissioned system

The calculator is step one of a short, well-trodden path. Step two is a desk feasibility, which an installer you choose can prepare: they take your half-hourly meter data and roof drawings and replaces the calculator's assumptions with your actual load profile — no site visit needed. Step three is a survey that converts the indicative band into a single fixed price. Step four is the decision on funding: capital purchase, asset finance, lease, or a power purchase agreement where someone else owns the roof asset and sells you the power. The funding comparison page runs the same 100kWp example through all four routes so you can see exactly what the no-capex options cost over a contract term. For a 25-year cash flow with payback, IRR, NPV and tax relief on your own numbers, use the commercial solar ROI calculator.

Whatever route you choose, walking in with your own numbers changes the conversation. You will know whether a quoted £1,250/kWp is out of band, whether a promised 4-year payback assumes an on-site use figure your site cannot deliver, and what your roof is actually worth over 25 years. That — not the last 5% of precision — is what a business solar calculator is for.