Most UK commercial buyers have never compared more than two commercial solar quotes. The variance between quotes from different installers — for the same project — frequently exceeds 35% on capex and 60% on projected savings. These are the seven things that separate a defendable quote from a marketing pitch.
1. The PVSyst yield model — or the absence of one
A real commercial solar proposal includes a PVSyst (or equivalent) yield model showing month-by-month modelled generation against the site's specific orientation, pitch, shading, panel specification, and inverter losses. Without this, the "you'll save £42,000 per year" claim is an estimate based on average UK irradiance, not your specific roof. Ask for the PVSyst output file. If the installer can't provide one, they haven't done the engineering.
2. Half-hourly meter data analysis
The single biggest variable in commercial solar payback is self-consumption — the percentage of generated kWh consumed on-site versus exported. Self-consumed kWh saves your full retail tariff (24-32p/kWh); exported kWh earns only the SEG export rate (4-15p/kWh). A proposal that models self-consumption against your actual half-hourly meter data is materially more accurate than one assuming sector-average load profiles. Request meter data analysis as part of the desk feasibility — not an extra service.
3. Fixed-price proposal (not "indicative" pricing)
Indicative pricing exists for change-order revenue. A defendable commercial proposal carries a fixed-price quotation valid for at least 60 days, with all line items priced — module type and quantity, inverter spec, mounting system, DC + AC cabling, isolation switches, scaffolding, DNO application fees, MCS certification, EICR commissioning, monitoring portal. Surprise post-install costs are how cheap quotes become expensive bills.
4. Tier 1 module specification — verified
BNEF Tier 1 status is the project-finance bankability standard. A real proposal names the specific module manufacturer and model (e.g. "JinkoSolar Tiger Neo 615 W TOPCon JKM615N-72HL4-V") not just "premium Tier 1 modules". The brand matters for warranty enforceability over the 25-year asset life. See our 2026 brand guide for the current Tier 1 list and which brand fits which project.
5. DNO position confirmed at quote stage
For any project above 100 kW, the DNO (Distribution Network Operator) connection process is the single biggest project-timeline risk. A good installer runs a DNO budget estimate (BE2) at the desk feasibility stage to confirm available capacity, expected reinforcement contributions, and Active Network Management (ANM) curtailment likelihood. A bad installer quotes the install price and ignores the DNO question — then surprises you with a £30,000 reinforcement contribution after contract.
6. Finance route comparison — not pre-baked
Most UK commercial buyers underwrite the project differently depending on whether they cash-fund with 100% AIA tax relief, asset-finance over 7 years, or go zero-capex via PPA. A good proposal models all three side-by-side so your finance director sees the real comparison. A bad proposal pushes one route based on the installer's referral commission with their preferred funder.
7. Year-one performance audit commitment
Real commercial solar projects deliver 100-104% of PVSyst-modelled generation in year one. Bad installs deliver 75-85% due to undetected shading, string-failure, monitoring portal glitches, or undersized DC cabling. A good installer commits to a year-one performance audit comparing actual generation against the modelled baseline, with remediation included if performance falls short of the warranty curve. Ask for this commitment in writing before signing.