Net present value answers a question payback cannot: is this project worth more than the capital tied up in it, once you account for when money arrives and your organisation’s cost of capital?
For UK commercial rooftop solar, NPV sits at the centre of most board-ready business cases. Estates teams may lead with payback; finance teams and investment committees usually want NPV — or both, with the discount rate stated explicitly.
What NPV means in practice
NPV is the sum of all future net cashflows from the project, each discounted back to today, minus the initial investment.
A positive NPV means the project is expected to create value above your hurdle rate. A negative NPV means the discounted costs outweigh the benefits — the roof may still save electricity, but not enough to justify the capex on financial grounds alone.
The formula is standard corporate finance. What matters for solar is not the algebra but the inputs: generation, self-consumption, export, tariff escalation, capex, operating cost, asset life, and residual value assumptions.
Those inputs should be traceable. Our methodology describes our financial approach; inputs are recorded in every dossier. The commercial solar financials hub places NPV alongside payback and IRR in context.
Building the cashflow that feeds NPV
NPV is only as good as the annual net benefit series behind it.
Year zero is typically negative: full capex, including installation, grid connection, and any allowance for enabling works flagged at feasibility stage.
Years one to twenty-five bring net operating benefit: avoided import cost plus export revenue, minus O&M, insurance, and major replacements such as inverter change-out around year twelve to fifteen on many designs.
Generation usually declines slightly each year through panel degradation — often modelled at 0.4–0.5% annually for modern modules.
Tariffs may be held flat, escalated with inflation, or stepped to reflect known contract end dates. Stating the basis avoids silent optimism.
Terminal value is sometimes included; many rooftop models simply run to year twenty-five with no residual asset value. Either approach is valid if documented.
The full structure is covered in how a 25-year solar cashflow model works. You can see a worked example in our example report.
Choosing a discount rate
The discount rate reflects the return your organisation requires to deploy capital. It is not universal.
A logistics group funding solar from operational budget may apply a different hurdle than a REIT evaluating solar against other asset improvements. Public sector bodies may reference HM Treasury Green Book guidance; private firms often use weighted average cost of capital or a fixed internal threshold.
For screening purposes, showing NPV at two or three discount rates — for example 6%, 8%, and 10% — helps a board see sensitivity without debating a single sacred number.
Feasibility is feasibility-grade investment analysis, not audited advice. The dossier should make that limit clear while still giving finance stakeholders numbers they can drop into their own models.
Why NPV beats payback for capital decisions
Simple payback tells you when cumulative savings cross capex. It does not tell you whether year-eight savings are worth as much as year-one savings, and it treats year-six breakeven the same whether the asset lasts ten or twenty-five years.
Two commercial roofs might both show seven-year payback. One exports most of its output at uncertain SEG rates; the other offsets expensive peak import on a manufacturing line. The second often produces higher NPV because the value per kWh is higher and more durable.
NPV also surfaces projects where payback looks long but lifetime value is strong — common on large roofs with moderate load match but low capex per kWp.
We compare NPV with IRR and payback in solar IRR vs payback for commercial projects.
Common NPV pitfalls on UK commercial roofs
Inflation double-counting. Escalating both tariffs and discount rate without care can distort results. Align nominal or real treatment across the model.
Export overstatement. NPV is sensitive to export income over 25 years. A feasibility study should tie export assumptions to a named tariff or conservative proxy.
Capex scope gaps. If grid reinforcement or structural upgrade sits outside the modelled capex, NPV is overstated. Engineering flags at commercial solar feasibility stage exist partly to surface these costs early.
Self-consumption guesswork. Using a sector average when half-hourly data is available weakens NPV without improving speed. Where data is missing, feasibility should say so and widen sensitivity.
Ignoring constraints. A roof that cannot export due to DNO limits may still have acceptable payback on self-consumption alone but lower NPV than an unconstrained scenario. Screening catches many of these before they reach the board paper.
NPV in the board paper
Investment committees typically want a one-page summary: capex, NPV at the corporate discount rate, payback, IRR, and key sensitivities — what happens if generation is 10% lower or if import tariffs fall at contract renewal.
A standalone NPV in a slide deck without assumptions is worse than no NPV at all. The commercial solar business case article covers how to assemble the narrative around the numbers.
Stage1Energy’s site assessment delivers a 29-page dossier including 25-year NPV with sourced workings at a £1,250 fixed fee per site. Not ready for the full report? Free screening returns a verdict in three working days so you know whether deeper financial modelling is justified.
NPV is not the only metric — but for UK commercial rooftop PV, it is usually the one that turns a promising roof into an approved project.
To model these metrics for a named UK commercial roof, see solar feasibility study cost in the UK or start with free screening.