The short answer
A good commercial solar installer can show you the right certification for the size of system you need, adequate insurance, clear warranties with someone standing behind them, a design based on your own consumption data and a properly surveyed roof, and a realistic plan for network approval and maintenance.
When comparing quotes, line them up item by item: system size, panels and inverters, mounting and roof work, generation per kWp, the share used on site, export price, grid connection costs, O&M, warranties and exclusions. The cheapest headline price is often the one that leaves the most out.
Certification: what applies at which size
Systems up to 50 kW: MCS
MCS's solar PV installation standard, MIS 3002, covers systems up to 50 kWp. MCS certifies both installation companies and products. For a system of this size, Ofgem's guidance says a supplier paying the Smart Export Guarantee will ask you to show that the installation and installer are suitably certified, for example with an MCS certificate. You can check an installer's certification on the MCS website. Ask for their certification number and check that the scope covers solar PV and, if you want one, battery storage.
Systems over 50 kW
Larger systems sit outside MCS's installation standard. For the Smart Export Guarantee, Ofgem's guidance says an installation between 50 kW and 5 MW must be suitably certified, with no installer certification requirement, and the supplier decides what evidence it needs. Our Smart Export Guarantee guide explains how export payments work.
Because there is no single certificate that proves competence at this scale, you need to look at the evidence more closely:
- experience of systems of a similar size and roof type, with references you can speak to
- a named designer who will take responsibility for the electrical and structural design
- design to recognised standards, including BS 7671 and the IET Code of Practice for grid-connected solar PV systems
- experience of G99 applications with your network operator
- understanding of RC62, the fire safety recommendations many insurers refer to
- the ability to act as principal contractor under the Construction (Design and Management) Regulations 2015 where required
Many installers who work on larger systems also hold MCS certification for smaller ones. That is a useful signal, but it is not proof of capability above 50 kW. Our page on commercial solar installers sets out the checks we make before matching one to a site.
Electrical competent person schemes
NICEIC and NAPIT are the best known electrical competent person schemes. Membership lets electricians self-certify notifiable work under the Building Regulations, which is mainly relevant to dwellings. On commercial work the more useful point is that members are regularly assessed on the quality of their installations. Ask who will carry out and certify the electrical work, whether that is the installer's own team or a subcontractor, and which scheme they belong to.
Consumer codes: RECC and HIES
RECC and HIES are consumer codes approved by the Chartered Trading Standards Institute. MCS certified installers must belong to one. Their protections, such as deposit protection and complaints handling, are designed mainly for domestic customers. For a business contract, your protection comes from the contract itself, so read the payment terms, warranty terms and dispute process carefully rather than relying on code membership.
Insurance
Ask for current certificates, not just a statement in the quote. The covers to check are:
- Public liability, for damage to your property or injury to others during the works
- Employers' liability, which is a legal requirement for most employers
- Professional indemnity, if the installer is responsible for design
- Contract works or contractors' all risks, covering the installation until handover
- Product liability, where they supply equipment under their own name
Check that the limits are proportionate to the value of your building and the project, and that subcontractors carry their own cover.
Warranties
Commercial solar involves several separate warranties from different parties. Find out what each covers and who you would claim against if the installer stopped trading.
| Warranty | Who usually gives it | What to check |
|---|---|---|
| Panel product warranty | Panel manufacturer | Length, what counts as a defect, whether it covers labour to replace panels |
| Panel performance warranty | Panel manufacturer | Commonly 25 to 30 years; the guaranteed output curve and how a claim is proven |
| Inverter warranty | Inverter manufacturer | Standard term and cost to extend; inverters are often replaced once in the panels' life |
| Mounting system warranty | Mounting manufacturer | Whether it depends on the installer following the manufacturer's design tool |
| Workmanship warranty | Installer | Length, what it covers, whether it is insurance backed |
| Roof warranty | Roofing manufacturer or contractor | Whether penetrations or overcladding affect an existing roof guarantee |
| Battery warranty | Battery manufacturer | Years and throughput limits, and the capacity guaranteed at the end |
Design documents you should expect
A professional quote for a commercial system comes with, or leads to, a set of design documents. Before you sign, you should see at least the first three. The rest normally follow during design and handover.
- A roof layout drawing showing panel positions, edge zones, access routes and roof plant.
- A generation report with the modelling software, location, orientation, tilt, shading and losses stated.
- An electrical overview, such as a single line diagram showing the inverters, protection, connection point and any export limitation.
- A structural assessment or engineer's letter confirming the roof can take the extra load.
- A fire safety approach, including DC isolator positions, cable routes and firefighter information, ideally referencing RC62.
- The G99 application or a plan for it, where the system exceeds the small-system limit. See G99 and DNO applications.
- Construction phase documents, including risk assessments and method statements.
- A handover pack with electrical certificates, PV test results, monitoring access, warranties and an O&M manual.
Yield assumptions: where quotes quietly differ
Two quotes for similar systems on the same roof can show very different savings. Usually the difference is in the assumptions, not the equipment.
Generation per kWp
A typical UK rooftop array produces roughly 800 to 1,050 kWh per kWp per year, higher in the south and lower in the north and Scotland. East-west flat roof layouts produce less per kWp. If a quote falls well outside that range for your location and layout, ask why.
Share used on site
The value of solar depends heavily on how much generation you use on site rather than export. Ask whether the self-consumption figure was calculated from your half-hourly data or assumed. Our page on energy data analysis explains why this matters.
Prices and inflation
Check the import unit rate used (it should be your own rate), the export price, and any annual energy price inflation assumed. A high inflation assumption makes every quote look better. Ask each installer to show results on the same price basis, and compare against our solar payback guide.
Degradation and losses
Panels lose a small amount of output each year, and the generation report should include an annual degradation figure, along with losses for cabling, inverters, temperature and soiling.
Operation and maintenance
Ask what happens after handover. Is monitoring included, and for how long? Who watches the alerts? What are the response times for faults? Is cleaning or thermography included, and what does O&M cost from year two? Our commercial solar maintenance guide explains what a sensible O&M arrangement looks like.
Questions to ask every installer
- Is your MCS certification number for solar PV, and does it cover this system size?
- For a system over 50 kW, which similar projects have you completed, and can I speak to those clients?
- Who is the named designer, and who carries out and certifies the electrical work?
- Was the self-consumption estimate based on my half-hourly data?
- What generation per kWp have you assumed, and why?
- What roof checks have you done, and is a structural assessment included?
- Does the price include scaffolding or access, G99 costs, any network reinforcement, and asbestos work?
- What warranties apply, and which are backed if you stop trading?
- What are the payment stages, and how is any deposit protected?
- What monitoring and maintenance is included, and what does it cost afterwards?
Red flags
- Pressure to sign quickly, or a discount that expires within days.
- A quote produced without looking at the roof or your consumption data.
- No mention of network approval for a system that clearly needs G99.
- Generation estimates well above the typical range with no explanation.
- Equipment described only as "tier 1 or equivalent" with no named models.
- No questions about asbestos on an older roof.
- Large payments up front that are not linked to delivery milestones.
- Reluctance to name the electrical contractor or provide insurance certificates.
Comparing quotes like for like
Put the quotes side by side in a table like this one, with a column for each installer, and fill in every row. Any blank is a question to send back.
| Item | What to compare |
|---|---|
| System size | kWp, number and model of panels |
| Inverters | Make, model, number, warranty term |
| Mounting | System type, ballast or fixings, structural sign off |
| Roof work | Repairs, overcladding or asbestos work included or excluded |
| Access | Scaffolding, edge protection or platforms included |
| Generation | kWh per year and kWh per kWp assumed |
| Self-consumption | Percentage used on site and whether based on your data |
| Prices | Import rate, export rate, inflation assumed |
| Grid connection | G99 application, export limitation, any reinforcement costs |
| Monitoring | Platform, length of included access |
| O&M | What is included and price from year two |
| Warranties | Panels, inverters, workmanship, roof |
| Payment terms | Deposit, stages, retention |
| Programme | Lead time, installation duration, disruption |
| Exclusions | Everything listed as not included |
Then compare cost per kWp only once the scope is equal. The shared illustrative ranges we use are about £1,000 to £1,500 per kWp for 10 to 50 kWp, about £750 to £1,100 per kWp for 50 to 250 kWp and about £600 to £900 per kWp for 250 kWp to 1 MWp, excluding VAT. Costs rise with difficult access, roof repairs, long cable runs, asbestos work or grid reinforcement.
How this fits with our survey
We match each suitable site with one MCS certified installer for a free survey, and your details go to that installer only. You can still use every question in this guide to test their proposal, and you are under no obligation to proceed. To prepare, see our survey checklist and how a survey works.
Frequently asked questions
Does an installer need MCS certification for a 200 kW system?
No. MCS covers solar PV up to 50 kW, so a 200 kW system sits outside the scheme. For Smart Export Guarantee payments the installation must be suitably certified, and the supplier decides what evidence it needs. Because no single certificate proves competence at that size, ask for comparable projects, references, a named designer, evidence of G99 experience and current insurance certificates. MCS certification for smaller work is a useful signal but not proof.
Do RECC or HIES protect my business?
Mostly not. RECC and HIES are consumer codes, and their protections such as deposit protection and complaints handling are designed mainly for domestic customers. MCS certified installers must belong to one, so membership is a sign of a regulated business, but your real protection on a commercial job comes from the contract. Read the payment, warranty and dispute terms carefully before signing.
How many quotes should we get?
There is no fixed number. What matters is that each quote is based on a proper survey and your own consumption data, so you are comparing real designs rather than estimates. If you do collect several, use a like for like table to compare system size, equipment, generation assumptions, inclusions, warranties and O&M. A blank row in one quote usually means something has been left out.
Why do two quotes for the same roof show different savings?
Usually because of assumptions rather than equipment. Check the generation per kWp each installer has used, whether self-consumption came from your half-hourly data, the import and export prices, any energy price inflation, and the degradation rate. Ask both installers to recalculate on the same price basis. Differences in system size and whether access, roof work or grid costs are included also change the result.
What documents should we receive at handover?
Expect electrical installation certificates, PV array test results, as-built drawings including a single line diagram, warranty documents for panels, inverters and workmanship, monitoring login details, the network operator's connection approval where relevant, and an O&M manual covering inspection, cleaning and safe isolation. Keep copies with your building records, because insurers, buyers and future maintenance contractors will ask for them.
Related pages
- Survey checklistA practical checklist of the energy data, drawings, consents and decisions to gather before a commercial solar survey, plus what to expect on the day and after.
- Solar maintenanceMonitoring, inspections, cleaning, thermography, electrical testing, bird proofing, roof access safety, O&M contracts and what insurers expect from rooftop PV owners.
- Fire safety and insuranceRC62 in plain terms, insurer questions, DC isolation, cable routing, separation from rooflights and fire breaks, and inspection regimes for commercial rooftop solar.
Commercial solar costIllustrative prices per kWp by system size, extra costs, running costs, VAT and tax.
Smart Export GuaranteeHow SEG export payments work for business solar in Great Britain, who is eligible, what accreditation and metering you need, and how much export adds to the case.- Installation processWhat happens between the first call and switch-on, who does each step and which stages take longest.