Reading a commercial solar quote — 12 things every finance director should check

A commercial solar proposal is a sales document. That doesn't make it dishonest, but it does mean it's been arranged to put the best foot forward — optimistic assumptions in the places you're least likely to check, conservative ones where it's safe to be cautious. The job, when one lands on your desk, is to find the assumptions and test them.

Here are the twelve things worth checking, roughly in the order they'll change your decision.

1. The assumed annual yield

Every proposal rests on how much electricity the system will generate. For a south-facing system in southern England, expect roughly 950 kWh per kWp per year. A 100 kWp system should therefore generate 90,000–100,000 kWh annually.

If the quote assumes materially more — say 1,050 kWh/kWp for a flat roof in Yorkshire — the payback figure is inflated. Ask what irradiance data and orientation they've assumed. A credible installer will show the modelling (most use PVGIS or MCS-standard figures).

2. The self-consumption rate

This is the single most-gamed number in commercial solar. Electricity you use on-site is worth your full retail rate (around 28p/kWh in 2026). Electricity you export is worth the much lower SEG rate. So the higher the assumed self-consumption, the better the payback looks.

Realistic commercial self-consumption is 65–80%. If a proposal assumes 90%+, ask them to justify it against your actual half-hourly consumption data. A building that's busy on weekday daytimes can hit the high end; one that runs evenings or weekends cannot.

3. The electricity price — and its assumed inflation

Check two things: the starting rate, and the annual increase they've baked in. A starting rate should match your actual contract, not a national average. An inflation assumption of 3–5% a year is defensible; anything above that flatters the long-term savings. If they've assumed 8% annual energy inflation for 25 years, the lifetime-savings headline is fiction.

4. The SEG (export) rate

In 2026, business export tariffs run from about 8.5p to 15p per kWh depending on supplier and whether the rate is fixed or variable. A conservative model uses 5–8p; 10–15p is achievable on the best fixed tariffs but assumes you actively switch and lock in. If a quote models export at 20p+, that's not real for 2026. Our guide to grants and funding covers SEG in more detail.

5. Installed cost per kWp

Divide the total system price by the kWp to get the cost per kWp. For UK commercial installs between 30 and 250 kWp, £800–£1,000 per kWp is the normal range in 2026. Below 20 kWp it's higher; very large arrays can be lower. A figure well above £1,100/kWp for a straightforward roof-mount deserves a question.

6. What's actually included

The headline price should cover panels, inverters, mounting, cabling, installation, scaffolding, DNO application, commissioning, and certification. The things that quietly get left out — and then appear as extras — are scaffolding for awkward roofs, electrical upgrades (a new DNO connection or switchgear), and structural surveys. Ask explicitly: "Is this price all-in, or are there likely extras after the survey?"

7. The inverter — and its replacement cost

Panels last 25+ years; inverters typically don't. Expect to replace the inverter at year 12–15, at a cost of £6,000–£15,000 for a system of this scale. A thorough proposal models this as a mid-life cost. Many don't, which quietly improves the lifetime return. If it's not in there, add it yourself.

8. The performance warranty, not just the product warranty

There are two different warranties. The product warranty covers the panel against defects (typically 12–25 years). The performance warranty guarantees the panel still produces a minimum percentage of its rated output after 25 years (usually around 85–87%). Check both, and check who honours them — a warranty from a manufacturer that may not exist in ten years is worth less than the paper it's on.

9. Capital allowances treatment

Commercial solar qualifies for the Annual Investment Allowance — a 100% first-year deduction. A serious proposal shows the post-tax payback, not just the gross. Be clear-eyed about what this means: a 100% deduction at the 25% corporation tax rate is worth about a quarter of the cost back in tax, not half. We cover the full tax picture in the grants and funding guide.

10. The MCS certification

For the installation to qualify for the Smart Export Guarantee, it must be MCS-certified, and the installer must hold a valid MCS registration. Ask for their MCS number and check it on the MCS website. This isn't bureaucratic box-ticking — it's the gateway to your export income and a baseline signal that the installer meets a recognised standard.

11. The payback figure — and which payback they mean

"Payback" can mean several things, and proposals rarely say which. Is it simple payback (cost ÷ annual saving) or discounted payback (accounting for the time value of money)? Is it pre-tax or post-tax? Does it include the inverter replacement? A 6.5-year simple pre-tax payback and an 8-year discounted post-tax-with-inverter payback can describe the same system. Ask them to define the number.

12. The contract: timelines, guarantees, and what happens if it underperforms

Finally, the commercial terms. What's the installation timeline and is there a penalty for slippage? Is there a generation guarantee — a commitment that the system will produce a minimum output, with compensation if it doesn't? What's the workmanship warranty on the installation itself (distinct from the product warranties)? And critically: what's the deposit, and what protects it if the installer goes under before commissioning?

The quick version

If you only have ten minutes with a proposal, check four things: the self-consumption assumption (should be 65–80%, not 90%+), the cost per kWp (£800–£1,000), whether the inverter replacement is modelled, and whether the payback is defined (simple vs discounted, pre- vs post-tax). Those four catch most of the optimism.

To pressure-test the headline savings against independent assumptions, run our calculator — it uses conservative commercial benchmarks, so if a quote's numbers are far rosier, you'll know where to push. And for the figures behind payback periods, read our guide to commercial solar payback in 2026.

For a monthly read on where prices, tariffs and standards are moving, subscribe to the Brief — independent, and written for buyers, not installers.