The real problem
Facilities directors often receive an installer proposal with a layout, a generation figure, and a payback - and ask whether it doubles as feasibility. It does not. Installers earn margin on hardware and labour, so the rational sales move is to propose the biggest system the roof will take. That is not always the system that maximises self-consumption or returns for you.
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.
Two documents, two incentives
Feasibility asks
What size is actually worth building?
- Right-size against your load profile
- Honest self-consumption vs export
- What could block the project
- Pursue, conditional, or stop - no sale attached
A quote asks
What would we install and charge?
- Often maximises roof fill (kWp = margin)
- Payback may assume flattering on-site use
- Commercial offer conditional on appointment
- Useful after the right size is confirmed
The sequence matters - see when to order a solar feasibility study. For estates working to an ESOS progress update or funding window, see UK energy compliance deadlines.
Scope and depth compared
| Aspect | Feasibility study | Installer quote |
|---|---|---|
| Primary incentive | Defensible verdict (including “stop”) | Win the install contract |
| System size | Sized to load, export limits, economics | Often fills usable roof |
| Self-consumption | Tied to stated load data | May assume high on-site use |
| Financials | NPV, payback, sensitivities | Capex, sometimes simple payback |
| Engineering / grid | Screening flags early | Often minimal unless asked |
| Verdict | Pursue / conditional / stop | Implicitly “proceed with us” |
| Cost | Fixed fee, no install to win | 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.
Why the biggest system is rarely free advice
Misaligned incentives
More kWp means more margin for them. Not always more value for you.
Commercial rooftop economics in the UK usually turn on self-consumption - displacing expensive imported power - not on exporting every spare kilowatt. An oversized array that dumps surplus at a weak export rate can look impressive on a proposal and still underperform a smaller system matched to daytime load. Installers are not paid to find that smaller system.
Independent providers charge a fixed fee - Stage1Energy’s site assessment is join the waiting list with written terms - and have no installation contract to win. Their incentive is a defensible verdict, including “do not pursue,” and a capacity recommendation that survives board scrutiny.
That does not make installers the enemy. Once feasibility has confirmed the opportunity and a sensible capacity band, competitive quotes are the right tool for price, programme, and constructability. The mistake is letting a sales layout set the brief.
- Use feasibilityTo decide whether to invite quotes - and at what capacity range.
- Use installer inputFor pricing and buildability against that brief, not to invent the brief.
- Cost of independenceSee solar feasibility study cost in the UK - a known fee before competitive quoting begins.
Where assumptions diverge
The largest gaps sit in assumptions readers rarely see on a quote PDF - especially size and self-use.
- System sizeQuotes often maximise roof fill because capacity drives contract value. Feasibility may recommend a smaller array if export limits or load profile favour it.
- Self-consumptionInstallers may assume high on-site use because it flatters payback on a large system. Feasibility should state load data sources and show how much value depends on export.
- TariffsFeasibility documents electricity price, export rate, and degradation. Quotes may use headline figures without sensitivity.
- Lifetime costsQuotes focus on installed price. Feasibility should acknowledge O&M, inverter replacement, and roof access.
- Risk flagsA quote may omit structural doubt, G99 complexity, or planning risk. Feasibility should surface them early.
Our methodology explains how assumptions are documented so readers can challenge them.
Recommended sequence
-
Filter marginal sites
Use waiting list when you need a fast first pass across candidates.
-
Commission feasibility
Confirm the case and a capacity band sized to load - before anyone fills the roof.
-
Brief installers
Quotes respond to the same capacity range and constraints - not open-ended roof fill.
-
Compare quotes
On scope, warranty, programme, and price - not on who proposed the largest array.
When to pause - and what to watch for
Installer-only may suffice
When the site is already well understood: recent solar on a sister building, known half-hourly load, confirmed structural capacity, and a single preferred contractor. Even then, an independent sanity check on size and self-consumption before board submission is cheap insurance.
Red flags on quote PDFs
Capacity that fills every square metre regardless of load or grid limits. Payback that assumes near-perfect self-use. No “do not proceed” outcome. Branded module datasheets without alternatives. Export assumptions missing. That is a sales layout, not an investment test.
For anything outside the narrow installer-only case - new building type, landlord consent, export uncertainty, or portfolio ranking - feasibility belongs first.
If you already have installer material, commissioning independent feasibility is not wasted duplication. It reframes those proposals against a common capacity brief and tells you which quote, if any, responds to a viable size - not just the biggest one.
Bottom line
A feasibility study and an installer quote are complementary, not interchangeable. Feasibility tests whether to spend - and at what size - without an incentive to upsell kilowatts. The quote is how much one party wants to build it. Start with commercial solar feasibility when you need that independent view.