What size commercial solar system does your business need?

Size commercial solar around the electricity your site can use while panels generate, then test roof and network constraints. PVGIS models production from an array rating in kilowatt-peak, including a 1 kWp reference system [S1]. Annual demand alone cannot show the overlap. Compare alternative layouts against interval consumption and a confirmed export limit before ordering equipment. Checked October 2026.

Last checked 16 October 2026

What size commercial solar system does your business need?

Begin with electricity use, usable space and connection constraints. Ask for an initial demand-led design and an alternative that uses more of the available roof. Compare what each produces, what your site uses and what remains for export. This gives you a decision about additional panels, rather than a single unexplained system size.

A kilowatt (kW) measures power. A kilowatt-hour (kWh) measures energy. Kilowatt-peak (kWp) is an array’s rated power under test conditions [S2]. Keep the panel rating, inverter power and annual generation on separate lines of the proposal. They answer different questions and should not be treated as interchangeable quantities.

Read solar costs after agreeing what the proposed capacity includes. A price without design scope cannot establish whether a larger system is useful for your site.

How do you calculate a starting solar capacity?

Use estimated generation per unit of installed capacity to translate an energy target into a trial array size. PVGIS can calculate average annual and monthly production for a grid-connected system [S1]. Use your actual location and proposed layout; the following arithmetic is an illustration of the method, not a measured yield.

All figures in this table are assumptions, not installation benchmarks or a quote.

The footprint is not the required roof area. Ask the designer to add row spacing, access and excluded zones to the measured drawing. Replace the assumed module dimensions with the selected product’s datasheet. Then have the roof assessment confirm that this specific layout is feasible before using it for procurement.

The trial array’s annual production equals the chosen target by construction. That does not establish how much your business will consume. A farm shop with refrigeration running continuously and an office closed at weekends need different demand inputs even if their annual electricity totals match.

How much generation will your site actually use?

Ask your supplier for interval consumption data and identify gaps, estimated readings and unusual closures. Provide the installer with operating hours, seasonal changes and known future loads. Keep a second demand file for proposed electrification so that uncertain expansion is visible instead of embedded in today’s base case.

For a spreadsheet comparison, align demand and generation to matching time intervals. In each interval, direct solar use is the smaller of generation and demand. Surplus is generation minus direct use, before storage and network limits. Sum the interval results to obtain annual totals. This is the calculation to request from the designer, rather than a fixed self-consumption percentage chosen to make a payback attractive.

PVGIS provides hourly production as well as monthly and yearly output [S2]. Ask how hourly generation has been matched to finer consumption data, and whether time zones and seasonal clock changes are handled consistently. Do not treat a model with mismatched timestamps as a precise site forecast.

For the assumed 45,000 kWh annual production above, an assumed 60% direct-use share means 27,000 kWh used and 18,000 kWh surplus. At an assumed 80% share, those quantities become 36,000 and 9,000 kWh. The arithmetic is generation × share, with the balance assigned to surplus. These are sensitivity cases, not observed site results.

Use self-consumption versus export to value those flows separately.

How do inverter size and export limits affect the design?

Ask for the array’s rated power, inverter output, permitted export and any export control settings. G99 is the engineering recommendation governing relevant generation connections; DNO means distribution network operator. Your DNO should confirm the route for the combined equipment. UK Power Networks describes separate eligibility checks for small generation and storage applications [S3]. Its process is an example, not a UK-wide connection offer.

Ask the designer to quantify energy lost when available generation exceeds equipment or export constraints. Record that curtailed energy separately from paid export. A battery proposal should show when spare capacity is available to absorb it; simply adding a battery capacity figure does not demonstrate that energy is captured.

The Smart Export Guarantee, or SEG, covers eligible generators in Great Britain, with supplier-set payment terms [S4]. Northern Ireland requires a separate export arrangement. Confirm the actual meter and contract before assigning income to the surplus column.

Should you fill the roof or leave space for expansion?

Request an incremental comparison. Show the extra installed cost, additional annual generation, extra direct use and extra export from the larger layout. Keep those additions separate from the original system’s result. If future charging demand justifies extra panels, ask when those vehicles will charge and how firm that demand is.

Use the roof survey guide to resolve structural constraints. Compare regional yield evidence with the actual location model. Have your accountant check any tax treatment before including it in a financial comparison.

What should you do next?

Prepare demand data, roof records and your planned load changes. Ask for alternative layouts with reproducible generation and consumption calculations before choosing equipment.

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