Choosing a commercial solar installer is a procurement decision, not simply a search for the lowest quote. This checklist shows UK businesses how to compare system designs, certifications, roof surveys, warranties, export assumptions, maintenance plans and whole-life costs using the same inputs for every supplier.
Overview
Commercial solar PV can reduce grid electricity purchases, provide a hedge against changing energy costs and support wider carbon-reduction objectives. The result depends on more than the panel capacity shown on the front page of a proposal. Roof condition, operating hours, electricity demand, shading, export arrangements, connection requirements and future changes to the building can all affect the business case.
A useful comparison starts by giving each installer the same information and asking each one to return the same core figures. Request the proposed system size, estimated annual generation, expected on-site use, exported energy, annual savings, capital cost, expected maintenance and key exclusions. If one quote includes a roof survey, grid works and monitoring while another does not, the headline prices are not directly comparable.
For a warehouse solar installation, farm solar panels or a system on a smaller commercial premises, treat the first quote as a design proposal rather than a final answer. The preferred installer should be able to explain the assumptions behind the design and identify what must be confirmed before contract signing.
How to estimate
Use a simple, repeatable model before comparing suppliers. It does not replace a detailed technical study, but it helps identify missing information and unrealistic assumptions.
- Establish annual electricity demand. Use at least 12 months of bills or half-hourly consumption data where available. Record annual kWh, tariff structure, standing charges and any significant seasonal pattern.
- Estimate usable roof or ground area. Exclude access routes, rooflights, plant, fragile areas, fire-safety routes and sections that may require future maintenance. Ask the installer to explain the usable-area calculation.
- Compare generation estimates. Ask for annual kWh generation, monthly output, panel orientation, tilt, shading assumptions and estimated system losses. A capacity figure alone does not show how much electricity the system may produce.
- Separate on-site use from export. A business normally gains more value from electricity used on site than from energy exported, but the exact difference depends on its electricity contract and export arrangement. Request both figures rather than accepting a single savings estimate.
- Calculate annual value. A basic model is: on-site solar used multiplied by the avoided import rate, plus exported solar multiplied by the applicable export rate, minus ongoing operating costs. Keep import and export rates as separate inputs so they can be changed later.
- Calculate simple payback. Divide the total project cost by estimated annual benefit. Label this as simple payback, because it does not fully account for degradation, financing, tax treatment, replacement components, inflation or changes in energy prices.
For a more complete procurement comparison, create a table with one row per installer and columns for system capacity, panel and inverter models, annual generation, self-consumption, export, capital cost, VAT treatment, roof works, grid works, warranties, monitoring, maintenance and exclusions. This makes vague differences visible.
Inputs and assumptions
Installer credentials and experience
Ask for the installer’s company details, relevant insurance, electrical competence, health and safety arrangements and examples of comparable commercial projects. Experience with a live warehouse, agricultural building or multi-roof site may matter more than a long list of small domestic installations.
Check whether the installer is currently MCS certified where that certification is relevant to the project, and confirm the scope of any certification rather than relying on a logo alone. Also ask who will carry out the electrical design, installation, commissioning and any remedial work. Certification and membership should be verified through the relevant organisation or register before relying on it in a procurement decision.
Roof, structure and site constraints
A credible quote should state whether it is subject to a roof survey or structural assessment. Ask about roof age, covering type, warranties, asbestos risk, wind loading, corrosion, drainage, access, lifting equipment and future reroofing. On agricultural sites, confirm how the design addresses dust, moisture, livestock environments and any restrictions on working hours.
For flat roofs, ask whether the design is ballasted or mechanically fixed and whether the load calculation, waterproofing and maintenance access are included. This guide to flat roof solar panels in the UK provides useful background. Ground-mounted proposals require separate consideration of land use, access, drainage, fencing and planning matters; see the guide to ground-mounted solar panels.
System design and components
Record the proposed panel technology, inverter arrangement, mounting system, cable routes, isolation equipment, protection, communications and monitoring. Ask whether the design uses one central inverter, several string inverters or another arrangement, and what happens if a component fails.
If battery storage, electric vehicle charging or backup power is being considered, require a separate calculation for each option. A battery can change the timing of electricity use, but its value depends on the site’s load profile, operating strategy, usable capacity, round-trip losses, warranty conditions and control software. Do not treat a battery as automatically improving the return of every commercial solar project. For background, see how to choose a solar battery in the UK.
Permissions, grid and contract scope
Ask the installer to identify any planning, landlord, lease, building-control, fire-safety or distribution-network requirements that may apply. The quote should state who is responsible for applications, fees, technical studies, export limitations, metering changes and remedial works. These items can materially affect programme and cost.
Check whether the proposal assumes unrestricted export or includes an export limit. If export income is included, ask which tariff or contract assumptions have been used and whether the business must arrange the export agreement separately. Treat any future tariff as an assumption to be tested, not a guaranteed return.
Warranties, monitoring and maintenance
Separate product warranties from the installer’s workmanship warranty. Ask who administers a claim, whether labour and access costs are covered, and what exclusions apply. Confirm the expected design life of the system, panel degradation assumptions and the proposed approach to inverter replacement.
Monitoring should show enough information to identify underperformance, including generation and system alerts. Ask who receives alerts, how quickly faults are investigated and whether remote diagnosis is included. Request a maintenance schedule covering inspections, electrical testing, cleaning where justified, vegetation control and repair call-outs. The relevant costs should appear in the financial model rather than being left as an unspecified future expense.
Worked examples
Consider a business comparing two proposals for the same building. Both installers quote similar system capacities, but Proposal A includes a roof survey, monitoring and a defined maintenance plan. Proposal B has a lower capital cost but excludes structural checks, grid-related work and access equipment. Proposal B may still be suitable, but its initial price is not its total project cost.
For each proposal, build an adjusted cost:
Adjusted project cost = quoted price + excluded mandatory works + expected enabling works + finance costs, where applicable.
Then model annual value using the same consumption data:
Annual value = (solar used on site × avoided import rate) + (solar exported × export rate) − annual operating costs.
If Proposal A forecasts more on-site use because it includes a different inverter layout or demand-management strategy, ask for the underlying half-hourly model. If Proposal B assumes that all generated electricity offsets purchased power, request a revised calculation. The most useful quote is not necessarily the one with the highest predicted savings; it is the one whose assumptions can be checked against the business’s actual operations.
For larger sites, ask for sensitivity cases. Recalculate the outcome with lower generation, reduced on-site consumption, a different import rate, no export income and a delayed commissioning date. This shows whether the project remains acceptable when conditions differ from the central forecast.
When to recalculate
Revisit the comparison whenever a key input changes. This includes electricity prices, export arrangements, finance rates, panel or inverter availability, roof condition, planning requirements, grid-connection terms and the business’s operating hours. Recalculate if the company adds machinery, changes shifts, installs EV chargers, introduces refrigeration or plans a building extension. These changes can alter both solar self-consumption and the appropriate system size.
Update the model at each procurement milestone: initial feasibility, after the roof and structural survey, after the grid response, before contract signing and before final approval. Keep a dated record of the assumptions used, including tariff rates and forecast generation, so later performance can be compared with the original case.
Before selecting an installer, obtain a final scope of works, programme, payment schedule, completion documents, commissioning procedure and named responsibility for defects. Compare at least the same information across shortlisted solar quotes in the UK, and use independent technical or legal advice where the project is material to the business. A disciplined checklist will not remove every uncertainty, but it will reveal which uncertainties remain and give decision-makers a sound basis for resolving them.
