Simple payback is a misleading metric for warehouse solar PV — but it's the headline number most installers lead with. IRR, after-tax cash payback, NPV, and LCOE matter more for accurate evaluation. This post explains why simple payback misleads and what to use instead.
What's wrong with simple payback
Simple payback is "capex divided by year-one savings". It ignores: time value of money (£1 today vs £1 in 10 years); tax shield from AIA / FYA / Freeport ECA; system degradation (0.4-0.5%/year); rising grid retail tariffs over the project life; and residual asset value at end of analysis period.
For a £900k install with £200k year-one saving, simple payback says 4.5 years. After-tax cash payback (after £225k AIA shield) is 3.4 years. NPV at 6% discount rate over 25 years is £1.8m. IRR over 25 years is 22.4%. LCOE (levelised cost of energy) is 4.8p/kWh — vs grid retail 22p/kWh. Each of these tells a different story; simple payback tells the worst story.
What we model
For every proposal we provide: (1) Modelled first-year generation in kWh from PVSyst; (2) Modelled self-consumption ratio from HH meter analysis; (3) 25-year DCF with explicit grid retail tariff and SEG escalation; (4) IRR, NPV, simple payback, after-tax cash payback, LCOE; (5) Sensitivity analysis under conservative grid price scenarios.
Why grid retail tariff trajectory matters most
The single biggest assumption in any solar DCF is grid retail tariff trajectory over the 25-year analysis period. UK grid retail has risen 40-80% since 2022 with TNUoS and BSUoS network charge increases compounding wholesale price volatility. Our base case assumes 4-6% annual nominal increase; conservative case assumes 2-3%. We always show the sensitivity.
Why simple payback still matters
Despite its limitations, simple payback is a useful "gut check" — does this make sense at first glance? Sub-5-year simple payback is excellent. 5-7 years is normal. Above 7 years deserves scrutiny.
UK warehouse solar economics 2026 — at a glance
UK commercial solar PV for warehouses has fundamentally changed economically between 2019 and 2026. Three structural shifts drive current 4-6 year paybacks: grid electricity has nearly doubled from 12-15p/kWh blended day rate in 2019 to 16-26p/kWh in 2026, with peak Time-of-Use rates now reaching 28-35p/kWh during 16:00-19:00 evening peak; battery system cost has fallen from £700-£900/kWh installed in 2020 to £250-£450/kWh in 2026; and 100% Annual Investment Allowance up to £1m of capex per year delivers immediate 25% corporation tax relief on solar capex. A typical 1 MW warehouse rooftop solar install costs £700,000-£800,000, generates 870,000-950,000 kWh per year, displaces £155,000-£180,000 of grid electricity annually, and pays back in 4-5 years before tax — 3-4 years after AIA tax shield.
Compliance pressure driving warehouse solar adoption in 2026
Four converging UK compliance forces make warehouse solar effectively necessary by 2030. (1) MEES EPC B by April 2030: all commercial property must achieve EPC rating B or better to be let. Solar PV adds 5-15 EPC points and is often the most cost-effective compliance route for warehouse stock currently at EPC C-D. (2) ESOS Phase 4 (December 2027 deadline): Energy Savings Opportunity Scheme requires large UK businesses to commission energy audits and implement or document rationale for solar recommendations. (3) SECR reporting: mandatory Streamlined Energy and Carbon Reporting requires Scope 1+2 emissions disclosure in annual reports — solar PV directly reduces reported Scope 2 figure. (4) Customer Scope 3 mandates: Amazon Climate Pledge, Tesco Net Zero, M&S Plan A, Sainsbury's Plan for Better, John Lewis Net Zero, JLR/Stellantis Tier-1 supplier programmes all flow Scope 3 supplier requirements through contract weighting and CDP/EcoVadis reporting. 3PL operators and owner-occupied warehouses serving these customers face direct commercial consequences if they fail to demonstrate verifiable renewable generation by 2027-2030.
How we model warehouse solar — half-hourly meter data, not assumptions
Every warehouse solar feasibility we deliver starts with your 12 months of half-hourly meter data and a roof drawing. Standard online solar calculators use generic per-sqft estimates that miss the operational pattern variation driving 30-40% of total payback difference. Our methodology: PVSyst yield model calibrated for your specific roof orientation, tilt and shading; self-consumption profile derived from your actual half-hourly demand at 15-minute resolution; 25-year DCF with monthly cashflow granularity; capital allowance schedule (AIA + ECA where applicable); grant funding scenario where eligible (IETF Phase 3 for manufacturers above 1 GWh/yr); SEG export tariff and REGO income; O&M cost schedule; sensitivity analysis on grid tariff inflation, self-consumption ratio, capex per kW and discount rate. Output: simple payback, after-tax payback, IRR, NPV at 4%/6%/8% discount rates, and 25-year cumulative return. If the numbers do not work for your specific site, we say so — we have walked away from over 60 projects since 2020 where economics did not justify proceeding.
Get a free desk feasibility — 7 working days
Send us 12 months of half-hourly meter data and a roof drawing (PDF or DWG). Within 7 working days we deliver: indicative system size from PVSyst modelling of your specific roof; financial DCF showing payback, IRR and NPV under three financing routes (outright purchase, asset finance, PPA); customer Scope 3 audit pack template for your supply chain context; grant funding eligibility assessment (IETF, UKSPF, Enterprise Zone ECA, Freeport ECA); DNO connection cost estimate from grid heatmap; structural pre-assessment from drawings; honest assessment of whether your site suits solar. No charge, no obligation. Call +44 7707 970 661 to discuss, or send your meter data to info@solarpanelsforwarehouses.co.uk — quote within 7 working days, guaranteed.