How to Compare Commercial Solar Quotes Line by Line
Updated 10 November 2026 · SEO Dons Editorial
How do you make quotes comparable before they arrive?
Give every bidder the same information, because quotes built on different assumptions cannot be compared line by line. CDM 2015 points the same way: the client must provide pre-construction information as soon as practicable to every contractor appointed or being considered for appointment. For rooftop PV that means the same set of papers to each bidder:
- twelve months of electricity bills, with half-hourly data if the meter records it;
- roof drawings, any structural information and the roof warranty position;
- the asbestos register and any survey reports. HSE says asbestos information should reach contractors as early as possible, for example when you ask for a quote;
- the electrical single-line diagram, meter details and any known network constraints;
- your tenure and any landlord restrictions.
Ask each bidder to price the same options, for example solar only and solar with a battery, and to show warranties and maintenance as separate lines. The feasibility study guide lists these documents in more detail. What drives the cost of the maintenance line, and what public award notices show, is set out in the commercial solar O&M costs guide.
What goes in the comparison table?
Copy this into a spreadsheet and fill one column per bidder. A blank cell is information too: it means the quote does not say.
| Line | What to check | Quote A | Quote B | Quote C |
|---|---|---|---|---|
| Capacity | DC peak power in kWp, and modules multiplied by wattage equals it | |||
| Modules | Make, model, quantity, wattage, datasheet attached | |||
| Inverters | Make, model, number, AC rating, the array kWp divided by inverter AC kW, and any clipping loss assumed | |||
| Modelled yield | Annual kWh and kWh per kWp, software, weather data, losses, shading | |||
| Your PVGIS figure | Same location, slope and orientation (see below) | |||
| Self-consumption | The percentage claimed and the demand data behind it | |||
| Connection and export | Route, who applies, who pays, export limit assumed, what changes if the offer differs | |||
| Roof and structure | Structural appraisal in or out, fixing method, roof warranty position | |||
| Access and asbestos | Scaffold, lifts, laydown, and what survey work is assumed | |||
| Electrical works | Board changes, cable routes, planned shutdowns, metering | |||
| Monitoring and O&M | Monitoring level, who owns the login, visit scope and term | |||
| Warranties | Module product and performance, inverter, workmanship, backing policy | |||
| CDM roles | Who acts as principal designer and principal contractor, if needed | |||
| Programme | Site duration, and what depends on the network operator | |||
| Payment | Milestones, what triggers each, retention and release date | |||
| Exclusions | Listed in writing, and how long the quote stays open |
The PV systems guide explains what each specification line decides, and the warranties guide covers the warranty row.
How do you check a quoted yield against PVGIS?
PVGIS is the European Commission's online tool for solar radiation and PV output at a location. It can be used without registering, and it gives you a figure that does not come from a bidder.
- Enter the site location on the map or by address.
- Enter the installed peak power in kWp. PVGIS defines it as the power the manufacturer declares under standard test conditions.
- Enter the slope and orientation from the quote's roof layout. East-west arrays need one run per face.
- Leave system losses at the default or replace them with the quote's stated loss figure. PVGIS gives a default of 14% for overall losses.
- Read the annual output, divide by the kWp, and enter it in the table.
Read gaps as questions, not verdicts. PVGIS describes its horizon option as estimating shadows from nearby hills or mountains, so shading from plant, parapets or neighbouring buildings needs your own horizon file, or a quote that models it and says so. PVGIS also makes no specific calculation for bifacial modules, so a quote that adds bifacial gain should show its basis. The guide to reading a PVsyst report covers the bidder's side of the same model.
What should the connection and export lines say?
The Energy Networks Association says a generator above 3.68 kW per phase comes under EREC G99, and MCS says G99 Type A covers 0.8 kW up to but not including 1 MW. ENA describes a fast-track route in G99 where total generating capacity, storage included, does not exceed 32 A or 60 A per phase and, where required, an EREC G100 export limitation scheme is present.
A quote should therefore name the route it assumes, who submits the application, who pays any network charges, what export limit is assumed and what happens to the price if the offer differs. ENA says compliance is the customer's responsibility, so ask who signs the compliance forms. The G100 export limitation guide explains limits in detail and the SEG guide covers what exported units can earn.
Which items are most often left out of a quote?
| Item | Question to ask each bidder |
|---|---|
| Intrusive asbestos survey | Is it included? HSE says installation work that disturbs the fabric where asbestos could be hidden needs a more intrusive survey before work starts |
| Structural appraisal | Who commissions it, and what happens to the layout if it limits the array? |
| Network costs | Are application fees and any reinforcement charges in or out? |
| Board and switchgear works | Is the existing board assumed to have spare capacity? |
| Access and lifting | Which scaffold, platform or crane costs are assumed? |
| Monitoring and cleaning | Are subscriptions, visits and cleaning in the contract? |
| Planning checks | Who confirms permitted development, listed building or conservation constraints? |
How should payment milestones and retentions be compared?
Lay the schedules side by side and ask what evidence triggers each payment: delivery to site, mechanical completion, commissioning, receipt of the handover documents. Ask what share is retained, for how long and who holds it. A schedule that pays most of the value before the network operator has accepted the connection leaves you carrying the main risk.
Part 2 of the Housing Grants, Construction and Regeneration Act 1996 may help, though whether it applies to a given PV contract is a question for a solicitor. Section 105 lists, among construction operations, the installation in a building of fittings forming part of the land, including power supply systems. For contracts within the Act, section 109 gives a right to stage payments unless the work is specified or agreed to last less than 45 days, and section 108 gives a right to refer a dispute to adjudication.
Where can independent cost data help?
DESNZ publishes solar PV cost data, but its page says the data is taken from the MCS Installation Database, and MCS certification covers electrical systems up to 50 kW. It is useful for small systems and says little about a rooftop array of several hundred kilowatts. For larger systems the benchmark is the other bids, which is why the table matters. The cost guide covers what drives the price, and the guide to vetting an installer covers who to ask. To start a comparison, you can send an enquiry through the quote form.
Sources
- legislation.gov.uk: CDM Regulations 2015, regulation 4; Housing Grants, Construction and Regeneration Act 1996, section 105; section 108; section 109, checked 22 October 2026.
- Health and Safety Executive: Provide information to anyone who might disturb asbestos; Find out if asbestos could be present, checked 22 October 2026.
- European Commission Joint Research Centre: PVGIS user manual, checked 22 October 2026.
- Energy Networks Association: Connecting generation, checked 22 October 2026.
- MCS: Notifying DNOs; Becoming certified, checked 22 October 2026.
- GOV.UK (DESNZ): Solar photovoltaic (PV) cost data, updated 28 May 2026, checked 22 October 2026.
Common questions
How many quotes should a business collect?
Two or three bidders working from an identical brief usually show the spread. More quotes add noise if each is built on different assumptions. Spend the time on the brief and the comparison table instead.
Should the lowest compliant quote win?
Price is one column of the table, not the decision. A lower figure with a long exclusions list, a thin warranty or most payments up front can cost more once the gaps are filled. Compare the delivered specification and who carries each risk.
Can a quote differ from PVGIS and still be right?
Yes. Shading from nearby plant, different weather data and different loss assumptions all move the result. What matters is that the bidder can show the model and explain the gap, and that the explanation is specific to your roof.
Do all bidders need to propose the same equipment?
No. Compare the datasheets and warranty documents for each proposal rather than insisting on one brand. The comparison is on delivered output, documented warranties and how each design handles the connection and the roof.