How many solar panels do you need in the UK? This practical guide shows businesses and homeowners how to estimate a suitable solar PV system size using electricity consumption, roof space, panel wattage, shading, orientation and future demand such as electric vehicles or heat pumps.
Overview
Solar panel sizing is not simply a matter of covering every available part of a roof. A useful design balances annual electricity demand, the timing of that demand, available roof area, the expected output of the site and the budget for the installation.
For a business, the strongest starting point is usually daytime electricity use. Offices, workshops, warehouses, farms, retail premises and other commercial buildings may consume much of their power while the panels are generating. That can make the size of the system particularly relevant to the amount of solar electricity used on site, rather than exported.
A sizing estimate is not a final design. An installer still needs to assess the roof structure, electrical equipment, fire access, cable routes, grid connection requirements, planning considerations and any battery or EV charging equipment. Use the method below to create a sensible brief before requesting solar quotes in the UK.
How to estimate
The basic calculation is:
Required system size in kWp = annual electricity target in kWh ÷ assumed annual generation per kWp
For example, a business might want solar to produce 18,000 kWh per year. If its planning assumption is 900 kWh per installed kWp each year, the initial estimate is:
18,000 ÷ 900 = 20 kWp
The 900 kWh figure in this example is an assumption, not a universal UK benchmark. The appropriate planning yield depends on location, roof pitch, orientation, shading, panel layout, system losses and the installer’s modelling method. Ask each supplier to state the yield assumption used in its proposal so that quotes can be compared consistently.
Once you have an estimated system size, divide it by the proposed panel rating:
Number of panels = required system size in kWp ÷ panel rating in kWp
If the proposed panels are rated at 500 watts, each panel is 0.5 kWp. A 20 kWp system would therefore require approximately 40 panels. The final count may change because installers must work with roof geometry, inverter limits, string design and safe maintenance access.
For a commercial site, begin with at least 12 months of half-hourly or monthly electricity data if it is available. Annual consumption tells you how much energy is used; interval data shows when it is used. That distinction matters when estimating self-consumption, battery value and export income.
Inputs and assumptions
Annual electricity consumption
Use bills, meter data or an energy management system rather than a guess. Separate unusual periods from normal operation, such as a temporary closure, a one-off construction project or abnormal production levels. If the premises has several meters, combine them carefully and confirm whether any supplies serve different buildings.
Future electricity demand
Include planned changes that could materially affect consumption. Examples include an electric vehicle fleet, workplace chargers, heat pumps, refrigeration, machinery, extended operating hours or a building expansion. Do not automatically size the array to every possible future load: record the expected timing, power requirement and installation date, then ask the installer to model staged options.
Roof area and panel dimensions
Panel wattage alone does not determine how many panels will fit. Two products with similar ratings may have different dimensions. Allow space around roof edges, access routes, rooflights, vents, skylights and fire-safety features. Flat roofs may need spacing between rows to manage shading and provide maintenance access. Read more about flat-roof solar panels before treating the full roof footprint as usable.
Orientation, pitch and shading
A south-facing roof is not the only viable option. East- and west-facing areas can produce electricity across different parts of the working day, which may suit a business with morning or afternoon demand. However, multiple roof orientations can affect inverter design and output modelling. Trees, neighbouring buildings, plant equipment and parapets should be assessed, including seasonal shading.
Panel rating and system components
Higher-rated panels can reduce the panel count for a target system size, but they may be larger or more expensive. Compare the complete design rather than selecting panels by wattage alone. The proposal should explain the solar inverter, mounting system, monitoring, warranties, cable routes and any battery or EV integration. A commercial solar installer checklist can help structure that comparison.
Export, batteries and connection limits
Solar output does not automatically match demand. Surplus generation may be exported, stored in a battery or diverted to a suitable load. A battery can change the value and timing of generation, but it does not remove the need for accurate load data. Check the proposed export arrangement and grid connection requirements rather than assuming the largest possible array can be connected without further assessment.
Worked examples
Small commercial premises
Suppose a workshop uses 9,000 kWh of electricity a year. The owner wants an initial design that targets 70% of annual consumption and uses an illustrative planning yield of 900 kWh per kWp.
Target generation: 9,000 × 70% = 6,300 kWh.
Estimated system size: 6,300 ÷ 900 = 7 kWp.
With 500-watt panels, the estimate is 14 panels. The installer should then test whether 14 panels fit without shading and whether the daytime load is sufficient to use a meaningful share of the generation.
Warehouse with future EV charging
A warehouse currently uses 36,000 kWh a year, with a proposed EV charging installation expected to add 12,000 kWh. The planning demand is therefore 48,000 kWh. If the business initially targets 60% of that demand and uses the same clearly stated illustrative yield:
Target generation: 48,000 × 60% = 28,800 kWh.
Estimated system size: 28,800 ÷ 900 = 32 kWp.
At 500 watts per panel, this would be approximately 64 panels. A roof survey may reduce that number, while a ground-mounted system or a second roof could provide additional capacity. See the guidance on pairing EV chargers with solar panels when charging demand is part of the business case.
Why the target percentage matters
Designing for 100% of annual consumption does not mean the site will run on solar all year. Generation varies by season, and a business may use electricity outside daylight hours. A smaller system may offer a better balance between capital cost, self-consumption and roof constraints. Compare several scenarios rather than treating one calculated panel count as the answer.
When to recalculate
Revisit the estimate before requesting final quotes and whenever an important input changes. Recalculate if annual consumption changes, operating hours move, an EV fleet or heat pump is added, a roof is replaced, shading develops, a building extension is planned or a battery is introduced.
It is also sensible to review the calculation when panel specifications, installation costs, electricity rates, export arrangements or finance terms change. These inputs can alter the preferred system size even when the roof itself remains unchanged. For financial modelling, keep the sizing calculation separate from payback assumptions and update both when rates or project costs move. The solar panel payback calculator provides a useful next step.
Before choosing a supplier, prepare a short data pack containing 12 months of consumption, half-hourly data if available, roof drawings or photographs, operating hours, planned electrification projects and any access constraints. Ask each installer to provide panel count, total kWp, estimated annual generation, monthly output, self-consumption, export assumptions, inverter capacity and the reason for any difference from your estimate.
Finally, treat the result as a design range rather than a fixed number. A well-prepared commercial solar brief might compare a roof-limited option, a consumption-led option and a future-demand option. That gives decision-makers a clearer basis for reviewing solar panel costs, battery storage and installation proposals without relying on panel count alone.
