Estates directors ask for payback. Finance directors ask for IRR. Both are reasonable — and on commercial rooftop solar, they are answering related but not identical questions.
Payback counts the years until cumulative net savings exceed upfront cost. Internal rate of return is the discount rate at which the project’s net present value equals zero — effectively, the annualised return the cashflow profile implies over the modelled life.
For UK commercial PV feasibility, presenting both metrics — with NPV at your corporate hurdle rate — is standard practice in board-ready papers. The skill is explaining when they align and when they diverge.
How each metric is derived
Payback uses undiscounted annual net benefits. Year-one savings count the same as year-fifteen savings. The calculation stops at breakeven unless you also report discounted payback, which fewer boards request.
IRR uses the full multi-year cashflow, including year-zero capex and ongoing O&M. It answers: if we treat this as an investment, what return does the profile imply?
Both depend on the same inputs: generation, self-consumption, export, tariff assumptions, capex, degradation, and replacements. Garbage in produces two different flavours of garbage. Transparent feasibility — see our methodology — matters more than which metric you lead with.
The commercial solar financials hub situates IRR and payback alongside NPV in a single framework.
When payback and IRR agree
On many owner-occupied industrial roofs with strong daytime load, sensible capex, and stable import tariffs, payback of six to eight years often pairs with IRR that clears typical corporate hurdles. The cashflows are relatively even; breakeven arrives mid-life; post-breakeven years add substantial value.
In those cases, leading with payback is fine — provided IRR and NPV appear in the appendix for finance reviewers.
When they diverge — and why it matters
Divergence is where projects get mis-sold or mis-rejected.
Short payback, lower IRR. Heavy year-one to year-five savings from high self-consumption on expensive peak power can pull breakeven forward. If export dominates later years at lower p/kWh, IRR may be less impressive than payback suggests. The project may still be worth doing; payback just flatters it.
Long payback, strong IRR. Large roofs with moderate load match may take ten or more years to break even on simple payback, but 25 years of steady savings can produce attractive IRR and NPV — especially if capex per kWp is low. Payback alone might kill a good project.
Tax and accounting treatment. Corporation tax, capital allowances, and balance-sheet classification can change the effective return for the entity approving spend. Feasibility dossiers typically present pre-tax project cashflows; finance teams may layer tax effects in their own models. State that boundary in the paper so IRR is not compared against a post-tax hurdle by mistake.
Capex timing. IRR is sensitive to when costs hit. If grid reinforcement lands in year zero but export capacity ramps in year two, payback and IRR move differently.
Landlord structures. If the landlord fronts capex but savings flow to tenants without a recovery mechanism, the landlord’s payback and IRR look poor even when the site is energetically strong. The business case must match the entity making the decision.
Understanding self-consumption dynamics helps explain many divergences — see self-consumption and export for commercial solar.
IRR limitations boards should know
IRR is not flawless. Multiple sign changes in cashflow can produce multiple IRR solutions — uncommon on straightforward solar but worth noting on complex lease structures. IRR also assumes reinvestment at the same rate, which may not match reality.
That is why experienced papers present IRR, NPV, and payback together, plus sensitivity tables. Commercial solar NPV explained covers discount rate choice; 25-year solar cashflow model shows the annual series beneath all three.
What to present at feasibility stage
At screening, metrics are feasibility-grade — not audited investment advice. They should still be:
- Computed from the same cashflow model
- Shown with stated assumptions
- Accompanied by engineering flags that might change capex or output
Installer quotes often headline payback because it is easy. Independent feasibility should resist that temptation and give finance stakeholders the full set.
Review the example report for a worked UK commercial roof with payback, NPV, IRR, and visible workings in one dossier.
Using metrics in portfolio ranking
When comparing five warehouses, IRR helps rank sites on return per pound of capex. Payback helps identify sites that recover cash quickly — relevant if capital is constrained or the organisation prefers shorter exposure.
Consistent modelling across sites matters more than the metric chosen. Mixed installer proposals with incompatible assumptions make both IRR and payback meaningless.
Commercial solar feasibility on a portfolio basis applies the same standard to every address so ranking is defensible.
Stage1Energy output
Every site assessment includes payback, 25-year NPV, and IRR with sourced assumptions — part of a 29-page dossier at £1,250 per site, delivered in five working days.
For an early view on whether metrics are likely in range, free screening returns a verdict in three working days.
Payback and IRR are not rivals. Used together, they give commercial decision-makers a fuller picture of what the roof is worth over its life — not just how fast it pays back.
If your organisation publishes a minimum IRR for energy projects, show whether the base case clears it under conservative self-consumption as well as central assumptions. That single extra line often determines whether finance engages constructively or sends the paper back for rework.
To model these metrics for a named UK commercial roof, see solar feasibility study cost in the UK or start with free screening.