Facilities directors and asset managers often receive an installer PDF with a layout, a generation figure, and a payback calculation. It looks like analysis. But the purpose of that document is to win your contract — not to test, independently, whether the roof is a sound investment.
Validating a commercial solar installer quote means separating what is technically plausible from what is commercially optimised for a sale. The checklist below is what estates teams use before recommending spend to a property director or investment committee.
For the structural difference between the two document types, see feasibility study vs installer quote.
Check the generation and yield assumptions
Ask for the specific yield figure (kWh per kWp per year) and the source — PVGIS, MCS guidelines, or proprietary modelling. Compare it to independent benchmarks for your region and roof orientation.
Red flags:
- Yield significantly above typical UK commercial ranges without explanation.
- Gross roof area used as array capacity without setbacks for plant, access, or fire routes.
- Shading dismissed without a layout that shows exclusions.
Generation drives every financial metric downstream. If yield is wrong, payback and NPV are wrong.
Check self-consumption and export
Installer quotes often assume a high self-consumption percentage because it shortens payback. Ask:
- What percentage was assumed, and what data supports it?
- Was load matched hour-by-hour or applied as a flat annual ratio?
- What export tariff or PPA rate was used for the remainder?
On lightly occupied buildings, export-heavy economics are fragile. See self-consumption and export for commercial solar for why load match matters more than roof area.
If the quote does not state assumptions, you cannot validate the payback — only repeat the headline.
Check what is in the capex figure
Installer pricing may exclude items that appear later:
- Grid connection and G98/G99 application costs.
- Structural remediation or additional loading works.
- Scaffolding, night works, or business interruption allowances.
- Monitoring, O&M, and inverter replacement over the modelled life.
Compare scope to a feasibility-grade capex build-up, not just the panel line item. Commercial solar payback explains what a complete denominator should include.
Check system size against the roof
Installers may maximise kWp to improve headline savings. Validate:
- Usable area after setbacks and obstructions.
- Whether the layout respects access routes and plant locations.
- Whether the proposed size triggers G99 and material grid works.
A smaller, well-matched array with strong self-consumption often beats an oversized system that exports at a discount.
Check engineering and grid risks
Quotes rarely include structural certification or formal DNO offers. At minimum, ask whether the proposal flags:
- Wind uplift and mounting suitability for your roof type.
- Structural loading on portal frames or fragile decks.
- Likely G98 vs G99 route and export constraints.
Feasibility addresses these at screening level with named next steps. Our engineering flags guide explains what should appear before you appoint specialists.
When three quotes disagree
Disagreement usually means incompatible assumptions — not that one installer is uniquely honest. Common causes:
| Variable | Why quotes diverge |
|---|---|
| System size | Different usable roof areas modelled |
| Self-consumption | 50% vs 80% with no load data |
| Capex scope | Grid works in or out |
| Product tier | Premium modules vs standard |
| Export rate | SEG, PPA, or unstated wholesale |
Without a common baseline, tender comparison rewards whoever assumed the most favourable inputs. Independent feasibility establishes that baseline before you negotiate.
When to commission independent feasibility
Order independent feasibility when:
- The board or capex committee needs a pursue-or-park recommendation, not three sales decks.
- You are comparing multiple sites and need consistent ranking.
- An installer quote looks attractive but assumptions are opaque.
- You want a documented park verdict if the case does not stack — without pressure to proceed.
Stage1Energy delivers a fixed-fee dossier in five working days, or free screening on one building in three — enough to know whether full assessment is justified before you spend on surveys or installer-led preliminary work.
Read what a commercial solar feasibility study is for scope, or open the commercial solar feasibility guide for the full process.