Facilities directors and asset managers often receive an installer proposal and reasonably ask whether it doubles as a feasibility study. The PDF may include a layout, a generation figure, and a payback calculation. It looks authoritative. But the purpose of that document is to win your contract — not to test, independently, whether your roof is a sound investment.
Understanding the difference between feasibility and a quote protects you from two common failures: pursuing weak sites because a sales deck was optimistic, and dismissing viable sites because one installer’s layout was unnecessarily constrained. If you already have a proposal in hand, use our checklist to validate a commercial solar installer quote before treating it as a decision document.
Different questions, different documents
Feasibility asks: Is this roof worth pursuing? What are the feasibility-grade economics? What could block the project? What should we verify next?
A quote asks: What would we install, what would we charge, and what are our payment terms if you appoint us?
Feasibility is decision support ahead of commitment. A quote is a commercial offer conditional on appointment. The sequence matters — see when to order a solar feasibility study.
Independence and incentives
Independent feasibility providers charge a fixed fee — Stage1Energy’s site assessment is £1,250 per site with written terms — and have no installation contract to win. Their incentive is to deliver a defensible verdict, including “do not pursue” when the numbers or flags do not stack up. Read about Stage1Energy for how independence is structured.
Installers earn margin on hardware and labour. Even ethical teams face pressure to propose capacity, product tiers, and timelines that favour a sale. That is not a criticism of installers; it is the nature of quoting.
Use installer input for pricing and constructability once feasibility confirms the opportunity. Use feasibility to decide whether to invite quotes at all.
Scope and depth compared
| Aspect | Feasibility study | Installer quote |
|---|---|---|
| Primary audience | Board, estates, asset manager | Procurement, project sponsor |
| Generation model | Indicative, method stated | Tied to proposed kit |
| Financials | NPV, payback, sensitivities | Capex, sometimes simple payback |
| Engineering | Screening flags | Mounting approach assumed |
| Grid / planning | Screening-level | Often minimal unless asked |
| Verdict | Pursue / conditional / stop | Implicitly “proceed with us” |
| Cost | Fixed fee | Usually free pre-appointment |
For what each feasibility section should contain, see what is in a solar feasibility report. Compare structure against our example report.
Assumptions that diverge
The largest gaps appear in hidden assumptions.
Self-consumption. Installers may assume high on-site use because it flatters payback. Feasibility should state load data sources — half-hourly meter data, benchmarks, or estimates — and show export dependence.
System size. Quotes often maximise roof fill. Feasibility may recommend a smaller array if export limits, load profile, or economics favour it.
Tariffs and inflation. Feasibility should document electricity price, export rate, and degradation assumptions. Quotes may use headline figures without sensitivity.
Lifetime costs. Quotes focus on installed price. Feasibility should acknowledge O&M, inverter replacement, and roof access — even if only at screening level.
Risk flags. A quote may omit structural doubt, G99 complexity, or planning risk unless it threatens the sale. Feasibility should surface them early.
Our methodology explains how assumptions are documented in every dossier so readers can challenge them.
Cost perception vs real cost
Installer quotes are often labelled “free.” The feasibility work is subsidised by expected contract value. That can be excellent value if you have already decided to proceed and are selecting between installers.
It is weaker value if you are still testing whether solar belongs on the building at all. You may receive three incompatible “feasibility” PDFs from three installers, each optimising for a different layout, and still lack an independent verdict.
Paid feasibility at a known fee — see solar feasibility study cost in the UK — buys a single baseline before competitive quoting begins.
How to use both in a sound process
A robust UK commercial rooftop process uses feasibility first, then quotes.
- Screen marginal sites with free screening if needed.
- Commission feasibility on sites worth serious consideration.
- Brief installers with the feasibility dossier so quotes respond to the same capacity range and constraints.
- Compare quotes on scope, warranty, programme, and price — not on whether solar works, which feasibility already answered.
If feasibility says pursue with conditions — for example, confirm structure before finalising capacity — bake those conditions into the tender.
When an installer-only path is acceptable
Installer-led assessment may suffice when the site is already well understood: you have recent solar on a sister building, known half-hourly load, confirmed structural capacity, and a single preferred contractor relationship. Even then, an independent sanity check before board submission is cheap insurance.
For anything outside that narrow case — new building type, landlord consent questions, export uncertainty, or portfolio ranking — feasibility belongs first.
Summary
A feasibility study and an installer quote are complementary, not interchangeable. Feasibility is the independent test of whether to spend serious money. The quote is how much one party wants to build it.
Start with commercial solar feasibility when you need that independent view. Bring installers in once the verdict supports it — and you will negotiate from a stronger position.
Red flags when a quote is dressed as feasibility
Watch for installer PDFs that lack a “do not proceed” outcome, use branded module datasheets without comparing alternatives, omit export assumptions, or present payback without NPV or sensitivity. Another warning sign is capacity that fills every square metre regardless of load or grid limits — a sales layout, not an investment test.
If you already have installer material, commissioning independent feasibility is not wasted duplication. It reframes those proposals against common assumptions and tells you which quote, if any, responds to a viable brief. That reframing often saves more than the dossier fee before you leave the meeting room.