KMM Energy Solutions· Build Your Own Power Station
Free tool · UK 2026

UK Commercial Solar Calculator — Honest Numbers, Bespoke Proposal

Calculate the realistic bill saving and payback for your commercial site. We use £0.75/W as the base capex anchor and 12p/kWh as the SEG export rate — both real market figures we apply in our financial modelling. The output is a range, not a sales number. Your bespoke design follows the Fully Funded Electrical & Structural Survey.

Tell us about your site

Six inputs. Results update live. We never store this data — it stays in your browser.

Picking your sector pre-sets a sensible day-vs-night split. Tweak it below if you have your own number.

Usable, unshaded roof area in square metres. We assume roughly 6.5 m² per kWp installed.

Your site’s total annual electricity spend, all months combined. We use this to back out your effective grid rate; leave the default if you’re unsure.

75% day

Higher = more of your usage happens 9 a.m. to 5 p.m. when solar is generating. Pulled from your sector by default.

A battery typically lifts on-site self-consumption from ~78% to ~92% by storing excess for evening use.

Your estimate

Estimated payback — range, not a single number
Without AIA
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With AIA (25%)
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System size
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Annual generation
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Annual bill saving (range)
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Implied grid rate —
SEG export income
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at 12p/kWh
Self-consumption
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Indicative capex — — bound to your bespoke design after the Fully Funded Electrical & Structural Survey.

Request a Fully Funded Survey → See what we deliver for commercial sites

How this calculator works

This calculator is built on the same constants we use inside our commercial financial model, so the numbers you see here move in the same direction the real proposal will. We anchor the capex at £0.75 per watt — the all-in turnkey rate KMM is delivering on commercial roof-mounts at 100–1,000 kWp scale in 2026. We assume the UK average yield of 950 kWh per kWp per year, which holds up well from Cornwall to Carlisle once you factor in pitch and azimuth.

Self-consumption defaults to 78% without a battery and 92% with one, modulated by the day-vs-night slider so a 24/7 manufacturing site doesn’t get the same answer as a 9-to-5 office. We export the rest at 12p/kWh, which is a fair mid-market 2026 SEG tariff. The payback you see is the band of likely outcomes — roughly ±15% around our central estimate — with and without the 25% effective Year-1 capex relief that the Annual Investment Allowance gives a CT-paying limited company.

What the survey adds

The calculator gives you the shape of the answer. The Fully Funded Electrical & Structural Survey gives you the bound, defensible answer your finance director can sign off against. It is fully funded by KMM — you pay nothing for the survey itself — and it adds four things this tool cannot:

  • Half-hourly data audit — we ingest 12 months of your HH meter data and run the actual self-consumption profile, not a sector average.
  • Drone roof survey — tilt, azimuth, shading, structural mounting zones, and panel-layout optimisation specific to your roof.
  • NGED / DNO capacity check — we run the grid-export application early so any G99 or capacity constraint surfaces before you commit.
  • Structural assessment — signed-off load calcs against your roof construction, including any cladding or purlin reinforcement that the design needs.

Frequently asked questions

How accurate is the KMM commercial solar calculator?
It is accurate enough to tell you whether commercial solar is worth a proper conversation — typically within ±15% of the cash model we produce after a Fully Funded Electrical & Structural Survey. The calculator anchors on £0.75 per watt, 950 kWh per kWp per year, 12p per kWh for SEG export, and 78% (no battery) or 92% (with battery) self-consumption. These are the same inputs we use in our internal financial model. The reason the on-site survey tightens the answer is that we then layer your real half-hourly meter data, the panel yield from a tilt/azimuth model of your specific roof, your DNO capacity check, and your tax position.
Why is the base capex £0.75 per watt?
£0.75/W (i.e. £750/kWp) is the all-in turnkey price KMM is delivering on commercial roof-mounts in 2026 at 100–1,000 kWp scale. It includes panels, inverters, mounting, DC and AC cabling, MCS and DNO paperwork, commissioning, and a 25-year workmanship warranty. Very small sites (sub-50 kWp) trend higher; very large sites (1 MW+) trend lower. We use it as the anchor in the calculator so the output sits on a defensible market number rather than a sales-led promotional figure.
Why do you show a payback range instead of a single number?
Because the single number is dishonest. Commercial solar payback depends on four variables that all move — your actual import tariff, your day-vs-night consumption split, electricity-price inflation, and how aggressively you choose to use the Annual Investment Allowance. The range you see is roughly ±15% around the central estimate, which is how confidently we can predict payback before we have your half-hourly data and a roof survey. Picking the midpoint will usually be close, but we would rather show you the honest band than over-claim a precise figure.
Does the calculator include the AIA (Annual Investment Allowance)?
Yes — it shows payback both before and after AIA. We model AIA as a 25% effective Year-1 capex reduction, which assumes you are a UK limited company paying the headline 25% Corporation Tax rate and you have enough taxable profit to absorb the deduction in the year you install. The full £1 million annual AIA cap covers solar systems up to roughly 1.3 MW at £0.75/W, so most KMM commercial installs sit comfortably inside it. If your company is loss-making this year, the deduction is carried forward; the effective relief still applies but lands later. Your accountant will confirm the exact treatment for your accounts.

See the bound, defensible answer for your site

The calculator is the shape. The Fully Funded Electrical & Structural Survey — fully funded by KMM — is the answer your board can sign off against.

Request your Fully Funded Survey →

Calculator constants: £0.75/W capex anchor · 950 kWh/kWp/yr yield · 12p/kWh SEG export · 78% / 92% self-consumption (no batt / with batt) · 25% effective AIA Year-1 relief. Outputs are indicative only and bound to the Fully Funded Electrical & Structural Survey. © KMM Energy Solutions 2026.