Due diligence checklist: what to verify before signing any commercial solar contract

Most of the risk in a commercial solar project isn’t in the panels — it’s in the paperwork. A system that performs well for twenty-five years is only as good as two things. First, will the company that installed it still be around in year five, when an inverter fails? Second, does the contract you signed match the terms the installer promised verbally?

None of the checks below take long. You can do all of them yourself, before you sign anything, without paying anyone for the privilege.

This is a working checklist, not a sales pitch for using an adviser. That said, it draws on the same due diligence we run for clients through procurement and tender support. Use it on a single quote, or to compare several.

1. Confirm the installer is who they say they are

“MCS-certified” is the baseline the UK market runs on. The Smart Export Guarantee requires it, and most funders and insurers won’t touch a system that lacks it. But a badge on a website or a van is only a claim, not proof. Verify it directly:

• Search the installer on MCS’s own register at mcscertified.com’s Find an Installer tool, by company name or postcode. If they don’t appear, or their status isn’t listed as currently certified, that’s a stop-and-ask moment — not necessarily a dealbreaker (some genuinely competent commercial contractors sit outside MCS at large scale), but you need the explanation before you proceed, not after.
• Know where MCS’s technical standard actually stops applying. MCS’s solar PV installation standard, MIS 3002, formally covers installations only up to 50kWp. Most commercial rooftop or ground-mount systems sit well above that. That doesn’t mean accreditation stops mattering — a credible commercial installer should still hold current MCS company certification and be able to point to a track record of systems at your scale — but it does mean the job itself needs specifying, commissioning and warrantying as a bespoke commercial project, not a scaled-up domestic one.
• Check any consumer-code membership for what it actually covers. Membership of the Renewable Energy Consumer Code or a similar scheme is a genuine quality signal, but these codes — and the insurance-backed guarantees that come with them — are built around protecting domestic consumers, not businesses buying at commercial scale. If your organisation is contracting as a business rather than an individual consumer, don’t assume that protection extends to you; check the contract itself instead (more on that below).

2. Check the installer’s financial standing

A twenty-five-year performance warranty is a promise from a company, not a law of physics. If that company doesn’t exist in five years, the warranty is only worth whatever’s left in the small print. That print usually names who else can honour it.

Companies House’s free register is the starting point, and it takes minutes:

• Filing history and accounts. Are accounts filed on time? Persistently late filing is a genuine early-warning sign trading standards and insolvency practitioners both watch for. For a small company you’ll likely only see abbreviated or micro-entity accounts (limited detail is normal and not itself a red flag), but a pattern of overdue filings, or accounts that have gone quiet altogether, is worth a direct question.
• How long the company has actually traded. A company incorporated eighteen months ago badging itself as “20 years’ experience” is either a rebrand, a change of ownership, or worth asking about directly. None of those are automatically disqualifying — but you should know which one it is.
• Charges and mortgages. The register shows registered charges against the company (loans secured against its assets). A heavily leveraged installer isn’t necessarily a bad one, but it changes what “the company folds mid-warranty” actually looks like for you as a creditor.

For any contract of real size, a Companies House search is only a first pass, not the whole job. It tells you what’s been filed. It doesn’t tell you whether the company can actually deliver, or stand behind a multi-year warranty.

That’s exactly why larger commercial contracts lean on contractual protection instead of trusting the counterparty to keep existing. One option is a parent company guarantee, where the installer is part of a larger group. Another is a collateral warranty, which lets you step in or claim directly if something goes wrong. On larger EPC-style contracts, a performance bond serves the same purpose.

That said, if a bidder can’t or won’t offer any of these on a six- or seven-figure contract, take note. That refusal is a data point in itself.

3. Read the warranty stack — there are four warranties, not one

Marketers treat “25-year warranty” as a single phrase, but a commercial solar contract actually carries several separate warranties. These come from different parties and run for different lengths of time:

Warranty	Typically covers	Typical length	Who’s actually on the hook
Product warranty	Manufacturing defects in the panels	12–20 years	The panel manufacturer
Performance warranty	Output not degrading faster than the guaranteed curve	25–30 years	The panel manufacturer
Inverter warranty	The inverter itself	5–12 years (often extendable)	The inverter manufacturer
Workmanship warranty	The installation — mounting, wiring, weatherproofing	Commonly 5 years or more	The installer

In practice, the gap most buyers miss is the inverter. Manufacturers typically warranty panels for far longer than the inverter that makes their output usable. Yet the inverter is also the part of a system most likely to need replacing within the warranty period.

Ask specifically what the inverter warranty covers and whether it’s extendable. Find out, too, whether you have to buy the extension at the point of installation, or can add it later.

Ask, too, who actually delivers on each warranty when the installer isn’t the manufacturer. That’s the normal arrangement.

In practice, a workmanship warranty is only as good as the installer’s continued existence (see point 2). Product and performance warranties, by contrast, sit with the manufacturer directly. That’s one reason established panel and inverter brands are worth the modest price premium. Favor ones with a real UK or European presence over an unfamiliar brand.

4. Check for a performance guarantee — a different thing from a performance warranty

A manufacturer’s performance warranty says the panels won’t degrade faster than a stated curve. It says nothing about whether the installed system — panels, inverter, wiring, shading, orientation — will generate the output the proposal promised. That’s what a performance guarantee, written into the contract itself, is for.

On larger commercial and EPC-style contracts, a performance guarantee typically commits the installer to a minimum guaranteed annual output. If the system underperforms against that guarantee, liquidated damages become payable. This is standard practice on genuinely large commercial and utility-scale solar contracts.

UK case law has already tested how these clauses hold up when a contractor fails to deliver. In those cases, the other party calls on the parent-company guarantee.

The equivalent for a smaller commercial rooftop system might be simpler: a written commitment to a first-year yield in kWh. You can then check the actual metered output against that figure.

Either way, watch for this gap. The proposal quotes an expected annual output, but the contract is silent on what happens if the system misses it. Close that gap before you sign.

5. What a fair contract looks like versus what should make you pause

Signal	Fair contract	Red flag
Specification	Detailed — panel model, inverter model, exact capacity, expected annual yield	Vague — “premium panels,” no named equipment
Payment terms	Staged against milestones (design sign-off, delivery, commissioning)	Large deposit upfront, balance not tied to a milestone
Cancellation	Clear terms either way, agreed in writing	No exit clause, or one so one-sided it’s unworkable
Yield claims	A stated methodology (irradiance data, shading analysis, degradation assumed)	A single confident number with no basis shown
Pressure to sign	None — you’re free to take the contract away and read it	”Today only” pricing, urgency to sign on the day
Sub-contracting	Named, disclosed if any part of the work is sub-contracted	Silent on whether the signing company does the actual installation
Financial backstop	Parent company guarantee, collateral warranty or bond on larger contracts	None offered, and the question is deflected rather than answered

None of this replaces reading the actual document. It’s a filter for knowing which pages to read twice.

Where this fits

Running this checklist on one quote is useful. Running it consistently across several competing bids is more powerful still. That only works with a specification precise enough to make the quotes genuinely comparable in the first place. Do both, and due diligence becomes a repeatable process rather than a one-off worry.

We built our procurement and tender support to close exactly that gap. We write the specification, manage the tender, and check every bidder against this list. That list covers accreditation, financial standing, warranty terms and contract structure. That happens before you’re the one signing anything.

If you haven’t yet established whether solar is worth doing at your site at all, that question comes first. It’s what our feasibility studies are for.