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Commercial Solar Payback Calculator (UK)

The short answer

Payback is the install cost divided by the yearly saving, and the saving depends most on how much of the output the business uses itself: a unit used on site saves the full unit rate, a unit exported earns only the export rate. Put in the site, the system size and the electricity use below. Every default is from PVGIS, DESNZ or another primary source, checked 23 October 2026.

The calculator

It opens on worked example 1 below. Change any box and the results update straight away.

Free calculator

Commercial solar payback

Runs in your browser. Nothing you type is sent anywhere or saved.

1. The site

Pick the nearest. The yield is PVGIS for that city.

PVGIS, south-facing at 35°. Lower it for east-west or flat roofs, or paste your design software's figure.

2. The system
Size it by
Install cost

Default: DESNZ mean for non-domestic installs of this size, 2025/26. Total £35,010.

3. The electricity
Annual use from

DESNZ average for a small (20–499 mwh) business, Q2 2026. Use the bill's rate if you have it.

SEG rates vary by supplier. 4.1p is a cautious published business rate.

4. When the power is used

Fills the two boxes below with typical starting assumptions. Edit them to match the half-hourly data.

Simple payback

5.6 years

Saving in year one

£6,273

Generation a year

29,040 kWh

25-year saving

£147,758

After the install cost

£112,748

CO₂e avoided a year

3.8 t

Where the 29,040 kWh goes

  • Used on site: 20,743 kWh (71%), saving £5,932 at 28.6p
  • Exported: 8,297 kWh, earning £340 at 4.1p
  • 25-year CO₂e avoided: 90 tonnes. Prices are held flat for 25 years, with output falling 0.5% a year.

How it works: the formulas

  1. Generation = system size (kWp) × yield (kWh per kWp), month by month using PVGIS's monthly profile for the region. From roof area, kWp = m² × 0.21.
  2. Used on site, each month = the smaller of (generation × days open ÷ 7) and (annual use ÷ 12 × daylight share). Generation on closed days, and anything above the daylight demand, is exported.
  3. Saving = kWh used on site × unit rate + kWh exported × export rate.
  4. Simple payback = install cost ÷ first-year saving.
  5. 25-year saving = the first-year saving added up over 25 years, each year 0.5% lower than the last if degradation is on. Prices are held flat.
  6. CO₂e avoided = generation × 0.13096 kg per kWh, the Government's 2026 average factor for UK electricity.

From a monthly bill, annual use = bill × 12 ÷ unit rate. DESNZ's prices are averages that include standing charges, so they slightly overstate what each unit saved is worth; the bill's own unit rate is better.

Assumptions and sources

InputDefaultSource
Yield (kWh per kWp a year)810–1156 by region, with a monthly profile (e.g. 1019 London, 853 Glasgow)PVGIS 5.3 at a city per region: south, 35° tilt, 14% losses
Install cost£1,650/kWp (0–4 kW), £1,320/kWp (4–10 kW), £1,167/kWp (10–50 kW); none above 50 kWpDESNZ solar PV cost data, non-domestic mean, 2025/26
Electricity unit rate38.4p / 28.6p / 27.7p / 25.2p / 24.4p / 22.7p / 22.2p by consumption band (smallest to largest)DESNZ QEP 3.4.2, Q2 2026, incl. CCL, excl. VAT
Export rate4.1p/kWhOctopus Energy's published SEG rate (suppliers set their own)
Grid carbon factor0.13096 kg CO₂e/kWhUK Government GHG conversion factors 2026
Panel degradation0.5% a year (can be switched off)NREL review of nearly 2,000 measured rates (median)
kWp per m² of panel0.21LONGi Hi-MO 5m: 415 W on 1.95 m² (0.2125), rounded down
Daylight share and days openOffice 60%/5 days, warehouse 50%/5, factory 55%/5, farm 50%/7, retail 60%/7, school 60%/5Typical starting assumptions, not data: edit them to match your half-hourly data

DESNZ's cost data comes from MCS installations, so it stops at 50 kW and includes VAT where it applied; above 50 kWp there is no default and the calculator asks for the quote. DESNZ notes that cost per kW falls as systems get bigger. All sources were fetched on 23 October 2026. For more UK business price and deployment figures, see TNW Marketing's UK commercial solar statistics.

What moves payback most

  • Self-consumption, first. At DESNZ's 28.6p for a small business against a 4.1p export rate, a unit used on site is worth about seven times one exported. A site open seven days with daytime processes beats an office shut at weekends, and an oversized system mostly adds cheap exports.
  • The unit rate. Smaller users pay more per unit (38.4p under 20 MWh a year in Q2 2026, 22.2p for the largest), so the same system pays back faster for them.
  • The install cost. Per kW it falls with size, which is why bigger roofs can still pay back well with lower self-use.
  • Yield. PVGIS gives 810–1156 kWh per kWp across the regions here; orientation, tilt and shading on the actual roof matter as much.

Two worked examples

Checked by hand and by a unit test against the same code.

1. 30 kWp office, Birmingham

  • 60,000 kWh a year at 28.6p; 60% in daylight, 5 days; export 4.1p
  • Generation 30 kWp × 968.1 kWh = 29,043 kWh
  • Used on site 20,745 kWh (71%), exported 8,298 kWh
  • Saving £5,933 + £340 = £6,273 a year
  • Cost 30 × £1,167 = £35,010
  • Payback 5.6 years; 25-year saving £147,773
  • CO₂e avoided 3.8 t a year

2. 50 kWp farm, near Exeter

  • 45,000 kWh a year at 28.6p; 50% in daylight, 7 days; export 4.1p
  • Generation 50 kWp × 1,052.2 kWh = 52,610 kWh
  • Used on site 22,385 kWh (43%), exported 30,225 kWh
  • Saving £6,402 + £1,239 = £7,641 a year
  • Cost 50 × £1,167 = £58,350
  • Payback 7.6 years; 25-year saving £179,999
  • CO₂e avoided 6.9 t a year

The farm's roof makes more power, but it uses well under half of it, so payback is two years longer. That is the self-consumption effect in one comparison.

Why a survey and half-hourly data beat any calculator

A calculator assumes. Half-hourly meter data shows when the site actually uses power: the overnight base load, the lunchtime peak, the shut-down fortnight in August. That decides self-consumption, the biggest number here. A site visit shows what the roof can carry, the shading, the condition and the connection: see our commercial solar survey checklist, the DNO application guide and the MIS 3002 performance estimate MCS asks for.

For installers, this is the opening, not the proposal: use it on the first call, then win the job with the survey. See how to sell commercial solar and capital allowances for solar. Solar Surveys UK books those surveys with business owners: pay per booked appointment, from £250 to £1,000+VAT by size and qualification, with no pitch, no pay (what commercial solar leads cost).

Commercial solar payback: FAQ

What is a typical payback for commercial solar in the UK?

It depends mostly on how much of the output the business uses itself. With this calculator's sourced defaults, a 30 kWp office in Birmingham comes out at 5.6 years and a 50 kWp farm near Exeter at 7.6 years, because the farm exports more. Put in the site's own figures rather than relying on a typical number.

How is solar payback calculated?

Simple payback is the install cost divided by the first year's saving. The saving is the electricity used on site times the unit rate, plus the electricity exported times the export rate. It ignores finance costs, inflation and tax relief.

Why does self-consumption matter so much?

A unit used on site saves the full unit rate: 28.6p for a small business on DESNZ's Q2 2026 figures. The same unit exported earns the export rate, 4.1p on Octopus Energy's published SEG tariff. So each kWh used on site is worth around seven times one exported.

Does the calculator include capital allowances?

No. A company buying the system can usually claim capital allowances and pay less corporation tax, which shortens the real payback, but the amount depends on its profits. Use the capital allowances calculator in our guide and let the customer's accountant decide.

What export rate should a business use?

Ofgem says SEG suppliers set their own rates, which must be above zero. The default here is Octopus Energy's published 4.1p SEG rate, a cautious figure. Octopus also publishes a 12p Panel Power rate for businesses that buy their electricity from it. Use the rate the customer can actually get.

How accurate is a solar payback calculator?

It is an estimate for a first conversation, not a quote. It assumes even monthly use, matched daylight demand and flat prices. A site survey and the business's half-hourly data show what the roof can take and when the power is really used, which is what decides self-consumption.

An estimate from public data, not financial, tax or engineering advice.

Sources

All opened and the figures taken on 23 October 2026.

Related guides

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