How to Compare Commercial Solar Quotes Like for Like

The total at the bottom of a solar quote is the least useful number on it. One installer may propose 120kWp to fill the roof, another 90kWp sized to your daytime demand. One may include scaffolding and the grid application, another may list them as extras. One may use premium panels and optimisers, another a simpler string design.

Each of those choices is legitimate, but they make totals impossible to compare directly. A £95,000 quote can be better value than an £82,000 one if it delivers more usable electricity, includes costs the cheaper quote leaves out, or uses equipment with longer warranties.

Before you look at price at all, check that every quote is answering the same question. Each should be based on the same consumption data, the same usable roof area and the same assumptions about how the building will be used. If one installer has your half-hourly data and another has only a single annual bill, their designs will differ for reasons that have nothing to do with quality. Send every installer the same information pack and you remove most of that noise at the start.

Divide each quote's total price, excluding VAT, by the system size in kWp. This removes the effect of different system sizes and gives you a like-for-like rate.

For example:

• Quote A: £82,000 for 90kWp, which is £911 per kWp
• Quote B: £95,000 for 110kWp, which is £864 per kWp
• Quote C: £88,000 for 100kWp, which is £880 per kWp

On this measure the most expensive quote is actually the cheapest per kWp. Most UK commercial installations in 2026 fall between £700 and £1,100 per kWp, with larger systems at the lower end. A quote far below that range deserves scrutiny of what is excluded; one far above it should come with a clear reason, such as a fragile roof or difficult access.

Price per kWp is a starting point, not a verdict. A system that is cheap per kWp but oversized for your demand will export much of its output for very little, which our commercial solar cost guide explains in more detail.

Two 100kWp systems on the same roof will not necessarily generate the same amount. Panel orientation, tilt, row spacing, shading, inverter choice and cable losses all affect output. Every commercial quote should state the predicted annual generation in kWh, and you can turn that into a comparable figure by dividing it by the system size.

This gives the specific yield in kWh per kWp per year. In the UK it typically ranges from around 850 in Scotland to about 1,000 to 1,100 in southern England for a well-oriented roof. East-west layouts on flat or low-pitch roofs produce less per kWp but fit more panels into the space.

If two installers propose similar layouts on the same roof but one predicts 15% more generation, that is not a better system. It is a more optimistic forecast. Ask each installer how they reached their figure, and treat any number well above the regional norm with caution until it is explained.

A generation forecast is only as good as its assumptions, and most quotes show the answer without the working. Ask every installer for the full simulation report, usually produced in software such as PVsyst or a similar design tool, and check these inputs:

• Weather data source: which irradiance database was used, and for which location.
• Performance ratio or system losses: the combined allowance for heat, soiling, cabling, inverter efficiency and mismatch. Unusually low losses inflate the forecast.
• Shading: whether nearby buildings, parapets, plant and rooflights were modelled.
• Degradation: the annual loss of output assumed for the panels, typically 0.4% to 0.5% a year for modern modules.
• Availability: whether the model assumes the system runs 100% of the time, with no allowance for faults or maintenance.

Then ask the question that settles it: will the installer stand behind the figure? Some offer a generation guarantee or a performance review after the first year. A forecast the installer will not put in writing should carry less weight in your decision than one they will.

Combining price and generation gives the most useful single comparison: what each unit of electricity will cost you over the system's life. A simple version is the system price divided by total generation over 25 years, allowing for degradation.

Using the earlier examples, with a typical yield of around 950 kWh per kWp:

• Quote A: £82,000 for about 2.0 million kWh over 25 years, roughly 4.1p per kWh
• Quote B: £95,000 for about 2.5 million kWh, roughly 3.8p per kWh
• Quote C: £88,000 for about 2.25 million kWh, roughly 3.9p per kWh

Grid electricity cost non-domestic customers an average of about 24p per kWh in early 2026, so all three are far cheaper than buying from the grid. The comparison between them is what matters for choosing an installer.

This figure ignores how much of the output you will actually use, and an exported unit is worth far less than one used on site. That is why the next step is checking the savings assumptions, not just the generation.

The savings forecast is where quotes diverge most, because it depends on assumptions the installer chooses. Check each one:

• Self-consumption: the percentage of generation the quote assumes you will use on site. It should come from your consumption data, not a standard figure. A warehouse closed at weekends cannot use 95% of its output.
• Electricity price: the import rate used should match your current contract, not a high or out-of-contract rate that flatters the savings.
• Price inflation: many models assume electricity prices rise every year. Ask for the payback with no inflation as well, so you can see the case without that assumption.
• Export rate: the price assumed for exported units should match a tariff you can actually obtain.

When quotes use different assumptions, ask each installer to rerun the model with the same inputs. A good installer will do this readily. If one refuses, that tells you something about how robust their figures are.

The cheapest quote is often cheap because of what it leaves out. Go through each quote and list any of these that are excluded or marked as provisional:

• Scaffolding, edge protection and access equipment
• Structural survey and roof load calculations
• Asbestos survey and fragile roof safety measures
• G99 application fees and any network reinforcement required by the DNO
• Upgrades to switchgear or the main distribution board
• Roof repairs found during installation
• Out-of-hours working if your site needs it
• Monitoring set-up and the first year of maintenance

Add a realistic allowance for each exclusion to that quote's price, then recalculate the price per kWp. It is common for the apparent cheapest option to move to the middle once exclusions are priced in.

Watch for the phrase "subject to survey" against large items. It is reasonable for a desktop quote, but the installer should give you an estimate range so the final price does not arrive as a surprise.

Each quote should name the exact panel and inverter models, not just the brands. That lets you check datasheets and warranty terms yourself.

• Panels: compare wattage, efficiency, product warranty (often 12 to 25 years) and performance warranty (often 25 to 30 years). Widely used manufacturers include LONGi, JA Solar, Trina, Jinko and Aiko.
• Inverters: compare warranty length and whether extensions are available. SMA, Huawei, Solis, Fronius and SolarEdge are all common in commercial work. Optimisers add cost but help where shading or complex roofs are involved.
• Mounting: check the system is rated for your roof type and wind zone.
• Workmanship: the installer's own warranty on the installation, and whether it is insurance-backed.

A manufacturer's warranty is only as useful as the route to claiming it. Ask who handles a fault, who pays for the labour to replace a failed part, and what happens to the workmanship warranty if the installer stops trading.

Accreditation protects your eligibility for export payments and your insurance, so confirm it for every shortlisted installer:

• MCS certification: required for Smart Export Guarantee payments on systems of 50kW or less. Above 50kW different evidence is accepted, but MCS remains a useful quality benchmark.
• Electrical competence: membership of a scheme such as NICEIC or NAPIT, so the electrical work is certified.
• Health and safety: on most commercial projects the installer acts under the Construction (Design and Management) Regulations 2015, so ask who will be principal contractor and request their method statement and risk assessment.

Experience matters as much as paperwork. Ask for two or three references from projects of a similar size and roof type, ideally completed a year or more ago so the client can comment on performance and aftercare. An installer who mainly fits 4kW home systems may not be the right choice for a 200kWp factory roof, however good their domestic work.

Put each quote into a simple grid with the same rows, filled in from the quote documents rather than the sales conversation, and the right choice usually becomes clear within a few minutes:

1. System size in kWp
2. Total price excluding VAT, plus an allowance for every exclusion
3. Price per kWp after exclusions
4. Predicted annual generation and kWh per kWp
5. Lifetime cost per kWh
6. Assumed self-consumption percentage and import price
7. Panel and inverter models and their warranties
8. Workmanship warranty and whether it is insurance-backed
9. G99 status and any export limit
10. Accreditations, references and proposed programme

The best quote is rarely the cheapest or the one with the highest forecast. It is the one with realistic assumptions, few exclusions and an installer prepared to stand behind the numbers. Read more about system options on our commercial solar page, or compare quotes from up to three MCS-certified commercial installers built on the same information.