Questions to Ask Your Solar Installer Before You Sign

Getting quotes from solar installers is exciting — but the difference between a smooth, high-performing installation and an expensive headache often comes down to the questions you ask before you sign anything. In the UK, the installer market ranges from highly professional MCS-certified firms with decades of experience to unaccredited sole traders with no insurance. These 12 questions give you a clear framework for separating the two.

1. Are you MCS-certified, and can I have your MCS number?

Why it matters: MCS (Microgeneration Certification Scheme) is the UK's quality mark for small-scale renewable installations. MCS certification is a legal requirement if you want to claim Smart Export Guarantee (SEG) payments for electricity you export to the grid. Without it, your installer may be perfectly competent — but you will be permanently locked out of export income.

Ask for the installer's MCS number and verify it yourself at mcscertified.com using the Find an Installer search. Confirm the scope covers solar PV specifically, as some installers are certified for heat pumps only. From May 2026, MCS is actively contacting customers to verify installations, so a legitimate installer will have nothing to hide.

2. Are you TrustMark registered?

Why it matters: TrustMark is the UK Government's only endorsed quality scheme for home improvement tradespeople. A TrustMark-registered installer has signed up to a code of conduct and customer charter, and their scheme operator carries out regular vetting. TrustMark registration is mandatory for any work funded under government energy-efficiency schemes such as ECO4, and it provides a layer of consumer protection beyond what MCS alone requires.

3. Which panels and inverter will you install, and what are the warranty terms?

Why it matters: Not all components are equal. Premium monocrystalline panels from established manufacturers typically carry a 25-year performance warranty guaranteeing at least 80% output at that point, plus a 10–15 year product warranty. Inverters usually carry a 5–10 year warranty, though this can often be extended. Ask the installer to hand you the product datasheets and warranty documents before you sign — a credible installer will have them ready.

If you want to understand the technical differences between inverter types, our guide to string inverters, microinverters, and power optimisers explains the trade-offs in plain English.

4. What is the estimated annual output, and how did you calculate it?

Why it matters: Any credible installer should be able to provide a projected annual yield figure in kilowatt-hours (kWh) and explain how they arrived at it. The industry standard is to use MCS-approved modelling tools (such as SAP or PVsyst) that account for your roof's orientation, pitch, shading, and local irradiance data. Be cautious of estimates that seem unusually high — they may be designed to make the financial case look more attractive than it really is.

To cross-check the numbers yourself, see our article on solar panel costs and payback periods in the UK.

5. What is the all-in price — including VAT, scaffolding, and DNO notification?

Why it matters: Solar panel installations currently attract 0% VAT for residential properties in Great Britain under HMRC rules. However, scaffolding, structural surveys, and electrical upgrades can add hundreds of pounds that some quotes deliberately omit. Ask for a fully itemised quote and confirm the total is the total — with no surprise charges on completion day.

6. Do you handle the DNO application (G98/G99)?

Why it matters: Before your system connects to the grid, your installer must notify your local Distribution Network Operator (DNO). Systems with inverters up to 3.68 kW per phase use the simpler G98 notification route; larger systems require a full G99 application, which can take up to 60 working days to approve. Reputable installers include DNO handling in their service and will not simply leave this task for you to navigate alone. Confirm it is included in writing.

7. What monitoring system is included?

Why it matters: A monitoring app lets you track daily generation, spot underperformance early, and build the data history you need to diagnose any future warranty claims. Most modern inverters come with manufacturer monitoring portals, but the quality varies significantly. Ask whether monitoring requires a subscription after the first year, and what happens to your data if you switch inverter brands.

8. What is your workmanship warranty, and is it backed by insurance?

Why it matters: MCS requires all certified installers to offer a minimum two-year workmanship warranty, but many professional firms provide five years or more. Crucially, ask whether the warranty is backed by an Insurance-Backed Guarantee (IBG). An IBG means that if the installer ceases trading — which does happen — the warranty is honoured by a third-party insurer rather than disappearing with the company. This is especially important given that solar systems are long-lived assets; your installer may not be in business in year seven when a roof tile works loose.

9. What happens if there is a roof leak after installation?

Why it matters: Roof penetrations and bracket fixings are the most common cause of post-installation problems. Under Section 49 of the Consumer Rights Act 2015, services must be performed with reasonable care and skill, and a trader is obliged to remedy any defects arising from their work. Ask explicitly: will you repair any roof damage at no cost, and is this covered by your public liability insurance? Get the answer in writing in the contract.

10. Can I see references or recent customer reviews?

Why it matters: An installer confident in their work will happily provide recent customer contacts or point you to verified review platforms such as Trustpilot or Google Reviews. The MCS Installations Database also records completed installations, so you can verify that the jobs they claim to have done actually show up on the national register.

11. How many days will installation take, and who will be on site?

Why it matters: A standard domestic solar installation (typically 4–6 panels up to a full 13-panel system) usually takes one to two days. If the installer quotes a much longer timeline or is vague about who will actually carry out the work, probe further — some firms subcontract to labour-only gangs whose skill levels may not match the accredited company you hired.

12. What are the payment stages?

Why it matters: Legitimate installers typically take a modest deposit (often 10–25%) with the balance due on satisfactory completion. Be wary of any firm requesting full payment upfront before work begins. Under the Consumer Rights Act 2015, you have the right to withhold final payment until the service is completed to a satisfactory standard. TrustMark-registered firms and those following the Renewable Energy Consumer Code (RECC) are required to ring-fence deposits and offer consumer protection in the event of trader insolvency.

Bringing it all together

Armed with these twelve questions, you are in a strong position to assess any quote objectively. A professional, MCS-certified installer will welcome your scrutiny — it demonstrates that you are a serious buyer. If an installer deflects, becomes vague, or cannot produce documentation for any of the above, treat that as a red flag and move on to the next quote. The right installer will make the process transparent from the first meeting to the day your system goes live.