Solar Panel ROI for UK Businesses 2026: Detailed Maths

UK commercial solar in 2026 typically delivers 12-18 percent IRR over a 25-year project life and 5-8 year simple payback. Strong cases — large industrial sites with 24/7 operations and full AIA capture — push past 22 percent IRR and 4-year payback. Marginal cases — short leases, night-only operations, low-tax-bracket businesses — fall below 10 percent. Working out which case you have is not guesswork; it is a discounted cash flow model with seven inputs and three outputs. This page lays out the methodology in full, runs four worked examples (50, 100, 250, 500 kW), shows how AIA reshapes the IRR, compares cash purchase versus asset finance versus PPA, and stress-tests against electricity price inflation.

The four ROI metrics that matter

Four numbers describe the financial case for any commercial solar system. Each tells you something different and a complete picture needs all four.

Simple payback period

Total capital expenditure divided by annual net cash benefit, in years. For a 50,000-pound system saving 9,000 pounds per year, simple payback is 5.6 years. Intuitive, easy to communicate, but ignores time value of money and the 19-20 years of generation after payback. Useful as a quick sanity check, not as the only metric.

Discounted payback period

Same calculation but applies a discount rate to future cash flows. For UK SMEs we typically use 7-9 percent as the discount rate, reflecting weighted average cost of capital. Discounted payback for a project with simple payback of 6 years lands at around 7.5-8 years at 8 percent discount rate. The right metric for capital-efficient comparison against other internal investments.

Internal rate of return (IRR)

The discount rate at which the net present value of all cash flows equals zero. Equivalent to the annualised effective return of the project. Typical UK commercial solar project IRR over 25 years lands at 12-18 percent for routine SME sites, 18-28 percent for large industrial 24/7 sites with full AIA capture. Compare directly to other capital allocation options: a 16 percent IRR project is materially better than a typical commercial property yield of 5-7 percent or a corporate bond at 5-6 percent.

Net present value (NPV)

The sum of discounted cash flows over the project life. For a 50 kW system on a typical UK SME at 8 percent discount rate, 25-year NPV typically lands at 60,000-110,000 pounds. NPV is the absolute pound value of the project — useful for boardroom decisions where the question is "what is this worth to us as a business" rather than "what is the rate of return."

The seven inputs that drive every ROI calculation

The DCF model needs seven numbers. Get any one wrong by 20 percent and the IRR moves by 1-3 percentage points. We model all seven against site-specific data before quoting.

1. Capex: total project cost ex-VAT (VAT recovered through normal returns and excluded from DCF).
2. Annual generation: kWh per year, modelled in PVSyst against satellite-derived irradiance for the site, accounting for shading, soiling, and array configuration.
3. Self-consumption ratio: percentage of generation used on site versus exported. Set by operating profile and load match.
4. Retail import price: current pence per kWh on your existing supply contract. Self-consumed kWh avoid this cost.
5. SEG export rate: pence per kWh paid by your export supplier for kWh exported to the grid. Currently 4-15p depending on supplier.
6. Electricity price inflation: assumed annual increase in retail import price. We model 2 percent conservative and 5 percent stretch scenarios.
7. Panel degradation: typical 0.4-0.7 percent per year linear after a 1-2 percent year-1 light-induced degradation. We use 0.5 percent per year as central case.

Three additional inputs apply on the financing side: corporation tax rate (currently 25 percent main rate, 19 percent small-profits rate up to 50,000 pounds profit), AIA capture (typically 100 percent for profitable companies under the 1 million pound annual cap), and discount rate (typically 7-9 percent for SMEs).

Worked example 1: 50 kW office, single-shift

14-person professional services office in Leeds, 35,000 pounds annual electricity (146,000 kWh at 24p), single-shift Monday-Friday 7am-7pm.

• Capex: 47,000 pounds ex-VAT
• Annual generation year 1: 47,500 kWh
• Self-consumption: 65 percent
• Annual saving: 7,410 pounds (self-consumed at 24p) + 998 pounds (export at 6p) = 8,408 pounds
• AIA tax saving year 1: 47,000 x 25% = 11,750 pounds
• Net capex after AIA: 35,250 pounds
• Simple payback on net capex: 4.2 years
• 25-year IRR (8% discount, 3% electricity inflation): 16.8%
• 25-year NPV: 96,000 pounds

Decision: comfortable case. The single-shift operation gives 65 percent self-consumption which is the lower end of the strong-case range; full AIA capture brings net payback well under 5 years. See offices for sector context.

Worked example 2: 100 kW retail and warehouse

Garden centre in Surrey, 22,000 sqft sales floor and warehouse, 75,000 pounds annual electricity (313,000 kWh at 24p), 7-day operation 8am-6pm with summer extended hours.

• Capex: 70,000 pounds ex-VAT
• Annual generation year 1: 92,000 kWh
• Self-consumption: 75 percent (high seasonal alignment of solar peak with summer trade peak)
• Annual saving: 16,560 pounds (self-consumed) + 1,380 pounds (export) = 17,940 pounds
• AIA tax saving year 1: 70,000 x 25% = 17,500 pounds
• Net capex after AIA: 52,500 pounds
• Simple payback on net capex: 2.9 years
• 25-year IRR: 22.4%
• 25-year NPV: 235,000 pounds

Decision: outstanding. Seasonal demand match with solar generation is a particular driver here — solar peaks in summer when garden centres peak. See 100kW solar system cost.

Worked example 3: 250 kW industrial unit, two-shift

Precision engineering business in 22,000 sqft industrial unit, West Midlands, 125,000 pounds annual electricity (520,000 kWh at 24p), two-shift 6am-10pm Monday-Saturday.

• Capex: 195,000 pounds ex-VAT
• Annual generation year 1: 220,000 kWh
• Self-consumption: 82 percent
• Annual saving: 43,296 + 2,376 = 45,672 pounds
• AIA tax saving year 1: 195,000 x 25% = 48,750 pounds (within 1 million pound annual cap)
• Net capex after AIA: 146,250 pounds
• Simple payback on net capex: 3.2 years
• 25-year IRR: 24.8%
• 25-year NPV: 690,000 pounds

Decision: strong case. Two-shift operation gives high self-consumption and AIA fully captured against substantial corporation tax. G99 application required (above 100 kW) — see G99 application and factories.

Worked example 4: 500 kW logistics warehouse, 24/7

Regional distribution warehouse in East Midlands, 60,000 sqft, 220,000 pounds annual electricity (917,000 kWh at 24p), 24/7 operation with refrigerated dock and conveyor systems.

• Capex: 395,000 pounds ex-VAT
• Annual generation year 1: 470,000 kWh
• Self-consumption: 91 percent
• Annual saving: 102,648 + 2,538 = 105,186 pounds
• AIA tax saving year 1: 395,000 x 25% = 98,750 pounds (within 1 million pound annual cap)
• Net capex after AIA: 296,250 pounds
• Simple payback on net capex: 2.8 years
• 25-year IRR: 27.2%
• 25-year NPV: 1,580,000 pounds

Decision: outstanding. Scale economics, very high self-consumption from 24/7 operations, AIA captured against substantial corporation tax. See warehouses and industrial solar panels.

How financing route changes the cash flow shape

For the 100 kW garden centre example above, here is how three financing routes change cash flow.

Cash purchase: 84,000 pounds inc-VAT day one (VAT recovered next quarter), 17,940 pounds annual saving, 17,500 pounds AIA tax saving year 1, 25-year IRR 22.4 percent. Largest day-one impact, highest lifetime return.

Asset finance over 8 years at 8 percent: zero day-one capex, monthly payment around 1,180 pounds, monthly saving from solar around 1,495 pounds, net cash flow positive from month 1 throughout the loan term and substantially positive thereafter. Equipment is owned at end of term. 25-year IRR around 14 percent on the business's actual cash invested, but the headline savings net positive throughout — important for finance directors prioritising cash conservation.

Power Purchase Agreement (PPA): third party owns and operates the system; business pays a discounted per-kWh rate (typically 12-16p versus 24p grid retail) for the energy generated. Day one capex zero, no AIA capture (third party gets it), no equipment ownership. Annual saving around 8,000-10,000 pounds versus grid retail at year 1. Long contract length (20-25 years) with index-linked rate increases. Right route for capex-constrained businesses or those without strong corporation tax position.

See finance options and commercial solar finance for full route comparison.

Inflation sensitivity

UK retail electricity prices have risen from around 12p per kWh in 2021 to 24p+ in 2026, an effective annualised rate of around 15 percent over five years. Future inflation is uncertain but the consensus view is for continued real increases driven by gas prices, carbon costs, and network charges. We model two inflation scenarios in DCF.

• Conservative case (2 percent inflation): 25-year IRR for the 100 kW garden centre example lands at 22.4 percent.
• Stretch case (5 percent inflation): 25-year IRR rises to 26.8 percent. Additional 25-year NPV uplift versus 2 percent: around 175,000 pounds.

Solar acts as a long-dated electricity hedge. The hedge value is real and often material to business decisions — particularly for energy-intensive sectors where electricity is a significant cost line.

The 25-year cash flow profile in detail

Solar ROI is shaped by a 25-year cash flow profile with five distinct phases. Understanding the shape helps when explaining the project to a finance director.

Year 0 (install): negative cash flow equal to the gross capex. For VAT-registered businesses, the 20 percent VAT is recovered in the next quarterly return so the effective cash impact is the ex-VAT capex.

Year 1: positive cash from generation savings (typically 5-15 percent of capex), positive cash from AIA tax saving for profitable companies (25 percent of capex on main rate, 19 percent on small-profits rate). Cumulative cash flow position improves dramatically — often the AIA tax saving alone covers a third to half of the gross capex.

Years 2-12: consistent positive cash from generation savings, slowly increasing as electricity inflation outpaces panel degradation. Net annual benefit typically grows from year-1 levels by 2-4 percent per year in nominal terms.

Year 12 (inverter replacement): typically a one-off negative cash flow of 80-150 pounds per kW for inverter replacement, partially offset by AIA-eligible capital allowance on the replacement. The 25-year DCF should model this explicitly.

Years 13-25: consistent positive cash flow continuing. Cumulative benefit by year 25 typically 4-8x the original net capex on strong-case projects.

Sensitivity analysis: what moves IRR most

Five inputs deliver the largest IRR sensitivity in the model. Tested by stress-running the 250 kW industrial example above.

• Self-consumption ratio: 75 to 90 percent moves IRR from 19% to 27%. The largest single sensitivity.
• Electricity inflation: 2 to 5 percent moves IRR from 22% to 28%.
• AIA capture: 100 to 50 percent (loss-making year) moves IRR from 25% to 19%.
• Capex per kW: 850 to 950 pounds per kW moves IRR from 25% to 22%.
• Annual degradation: 0.5 to 0.7 percent moves IRR from 25% to 23%.

The implication: the highest-leverage decisions in the project are operational profile and corporation tax position, not headline price negotiation. A 5 percent capex saving moves IRR by 1 point; getting self-consumption right moves it by 5-8 points.

What kills ROI — and how to spot it before signing

Wrong self-consumption assumption. Quotes assuming 80 percent self-consumption on a single-shift office that actually hits 60 percent overstate IRR by 4-6 points. Always demand a self-consumption number backed by half-hourly meter data, not a guess.

No AIA position check. A loss-making business or charity captures little or no AIA in year 1. Net capex is gross capex, IRR drops 4-6 points, and the case can flip from strong to marginal. Always confirm with your accountant before signing.

Underestimated degradation. A quote using 0.25 percent annual degradation looks better than one using 0.5 percent, but the 0.5 percent number is more realistic for tier-1 panels in real UK conditions. We use 0.5 percent as central case and run sensitivity at 0.7 percent.

Ignored short lease. A 4-year lease without portability cuts the project cash flow window from 25 years to 4. IRR tanks, payback exceeds the available time, the project does not work. PPA or operating lease structures sometimes recover the case; sometimes they do not. Always check lease length.

Hidden capex. A low headline price that excludes scaffolding, structural survey, asbestos survey, three-phase upgrade, or DNO admin is not a genuine project cost. See installation cost guide.

Authority resources

HMRC capital allowances rules: gov.uk Capital Allowances. Ofgem on SEG export tariffs: Ofgem SEG. UK government net zero strategy context: gov.uk Net Zero. MCS for installer competency: MCS.

Related decision pages

For the underlying buying decision see are commercial solar panels worth it. For installation cost detail see installation cost UK. For tax structure see capital allowances, VAT, and business rates. For financing see commercial solar finance, finance options, and grants. For sector-specific cases see factories, warehouses, offices. For DNO process see G98 or G99.