Before you sign a commercial solar contract, check five things. Most of the risk sits in the paperwork and the supplier, not the panels. You can do every check yourself, so none needs to wait until after you commit.
- Who the installer really is.
- Whether it can afford to stand behind its promises.
- What each warranty covers, and who pays.
- Whether there is a written performance guarantee.
- What the contract says about payment and sub-contracting.
Use the list on one quote, or run it across several bids, since that is the fairest way to compare them.
1. Check who the installer is
“MCS certified” is a common baseline in the UK, yet a logo on a van is a claim, not proof. Search for the company on MCS’s Find an Installer tool. If it is missing, ask why before you go on. Some capable contractors work outside the scheme, but you want the reason up front.
Know where the standard stops. MCS’s solar PV standard, MIS 3002, covers systems up to 50kWp. Most commercial systems are bigger. Hence the job needs its own spec, test plan and warranty, not a scaled-up home system.
While the standard stops at 50kWp, MCS’s Customer Commitment has a section on commercial contracts. It says that under one, an installer will:
- fulfil the terms of that contract
- say whether it will use sub-contractors
- provide a system performance estimate
- keep installation records for at least six years from the end of the relevant financial year
Ask each bidder to confirm these in writing. Take care with consumer codes too, since they aim at home buyers. They may not protect a business.
2. Check the installer’s finances
A long warranty is only a promise from a company. Because of that, if the firm is gone in five years, the promise may go too.
Companies House holds free data. GOV.UK says you can get information about a company. That includes officers, charges, insolvency details, previous names and filed documents. Look for these points:
- Filed accounts. Are they on time? Late or missing filings deserve a direct question. Small firms file limited detail, which is normal.
- Age of the company. If a firm set up 18 months ago claims “20 years’ experience”, ask whether it is a rebrand or a change of owner.
- Charges. Heavy secured borrowing is not always a bad sign. Still, it affects you as a creditor if the firm fails.
- Previous names and directors. These can show whether the business has traded under other names.
A search is only a first pass. On larger contracts, do not rely on the supplier surviving. Ask for a parent company guarantee, a collateral warranty or a performance bond. If a bidder offers none of these on a six or seven figure contract, ask why. Our guide to contract dispute remedies shows why these matter when things go wrong.
3. Read the warranties one by one
“25-year warranty” is a marketing phrase. A commercial contract holds several warranties from different parties. Each has its own length, so ask for each in writing.
| Warranty | What it covers | Who is on the hook |
|---|---|---|
| Product | Manufacturing defects in panels | The panel maker |
| Performance | Output falling faster than the stated curve | The panel maker |
| Inverter | The inverter itself | The inverter maker |
| Workmanship | Mounting, wiring and weatherproofing | The installer |
Lengths vary by maker and product, so read the documents and not a summary. Look hard at the inverter, because it is the part most likely to need work. Its cover is often shorter than on the panels. Ask if you can buy an extension, and whether you must buy it on day one.
Ask who honours each warranty if the installer disappears. The workmanship warranty is the weak point, because it is only as good as the installer. Our article on commercial solar insurance explains how an insurance-backed guarantee can fill the gap. For long-term output, see panel degradation and 25-year warranties.
4. Look for a performance guarantee
A maker’s performance warranty says panels will not fade faster than a set curve. Yet it does not promise that your whole system will hit the yield in the proposal, since wiring, shading and angle also matter. A performance guarantee in the contract does promise that.
On larger contracts it usually sets a minimum yearly output, with liquidated damages if the system falls short. For a smaller roof, a written first-year yield in kWh is a sensible match, because you can check it against the meter. Either way, put the measuring method in the contract.
5. Contract signals
| Signal | Reassuring | Warning sign |
|---|---|---|
| Specification | Named panel and inverter models, exact capacity, expected yield | “Premium panels” and nothing more |
| Payment | Staged against milestones | Large deposit, balance not tied to progress |
| Exit | Clear cancellation terms agreed in writing | No exit, or terms that are one-sided |
| Yield claim | A stated method, with irradiance data, shading and degradation | One confident number with no basis |
| Pressure | Time to take the contract away and read it | “Today only” pricing |
| Sub-contracting | Named and disclosed | Silent on who does the work |
| Financial backstop | Guarantee, warranty or bond on larger contracts | None offered and the question is dodged |
This table does not replace reading the document, although it tells you which pages to read twice.
6. Check the grid and roof early
A contract that ignores the grid link or the roof is incomplete, since either can kill a project. Ask who applies to the network operator, and who pays if the offer costs more than expected. Our guides to G99 grid connection and roof surveys cover both. A feasibility study settles them before you tender.
Making it repeatable
Running this list on one quote is useful, while running it across several bids is better. Each bid needs to follow a spec tight enough to compare. That is what our procurement and tender support does. We write the spec and run the tender. We also check every bidder’s certification, finances, warranties and contract terms before you sign. If you have not yet chosen between buying outright, a PPA or a lease, start with our comparison.
Due diligence works best with an independent view, because an installer has an interest in the answer. Our procurement and tender support builds these checks into the tender. A feasibility study sets the yield a guarantee should match, while PPA and financing advice reviews funded contracts and energy and ROI modelling tests the price. Public bodies should also read our guide to buying solar under the Procurement Act 2023.