What Size Commercial Solar System Do You Need? A Sizing Guide

Start with how much electricity your business uses during daylight hours, then design a system whose output you can use most of the time. For most businesses, the target is a system where 80% or more of the annual generation is consumed on site.

As a rough rule of thumb, each 1kWp of panels generates around 850 to 1,100 kWh a year in the UK, depending on location and orientation. A business using 100,000 kWh a year might therefore consider something in the region of 50 to 100kWp, but the right figure depends heavily on when that electricity is used. A factory running machinery from 7am to 6pm on weekdays can absorb far more solar than a business with the same annual total that uses most of its power at night.

Roof area and grid capacity then act as upper limits. If your demand would support 150kWp but the roof only fits 90kWp, the roof sets the size. If the roof fits 300kWp but your daytime demand supports 120kWp, demand should usually set it.

Accurate sizing needs to know when you use electricity, not just how much. Most businesses with larger supplies have half-hourly metering, which records consumption every 30 minutes. Your supplier, meter operator or data collector can usually provide 12 months of this data as a spreadsheet, often free of charge.

If you have a smaller supply with a smart meter, the data may be available through your supplier's online account. If you only have monthly or annual bills, installers can estimate a demand profile based on your type of business, but the result is far less precise.

What the data shows that bills cannot:

• Your minimum daytime demand on weekdays and at weekends
• How demand changes between summer and winter
• Peaks from specific equipment, such as compressors, ovens or chillers
• Whether you use electricity on bank holidays and shutdown periods

Send the same 12-month data file to every installer you ask to quote. It removes the biggest source of difference between proposals and makes their self-consumption forecasts directly comparable.

Baseload is the minimum level of demand your building draws during daylight hours, whatever else is happening. It typically comes from refrigeration, servers, ventilation, lighting, compressed air and equipment left running. Solar generation up to that level will almost always be used on site.

To find it, look at your half-hourly data for sunny weekday middays in May to August and note the lowest values. Then do the same for weekends. A distribution centre might show 80kW on weekdays and 35kW at weekends. An office might show 60kW during the week and 8kW on Sundays.

A system's peak output on a clear summer day is typically around 70% to 85% of its kWp rating, after losses. A 100kWp array might peak at around 75 to 85kW. If your weekday summer baseload is 80kW, almost everything a 100kWp system produces on working days will be used.

Weekend and holiday demand then decide how much is exported. A site with a much lower weekend baseload will export more on those days, which is why seven-day operations usually justify larger systems than five-day ones.

Many proposals size the system to fit the available roof, because a bigger system is a bigger sale and the lower price per kWp looks attractive. The extra panels, though, are the least valuable ones you can buy, because they generate mainly when your demand is already met.

Consider a warehouse with a weekday summer baseload of 60kW and room for 200kWp. As an illustration, a 100kWp system might see around 85% of its output used on site. Doubling to 200kWp might add roughly £80,000 and 95,000 kWh of generation a year, but perhaps only a quarter of that extra output would be used on site. At 25p for used units and 5p for exports, the second 100kWp would earn around £9,500 a year, a payback of more than eight years on that portion, against roughly four years for the first 100kWp.

The headline payback for the 200kWp system averages the two, so it can still look acceptable. Ask installers to show the marginal payback of each step up in size. It is the clearest way to see where extra panels stop paying their way.

Each 1kWp of panels needs roughly 5 to 6 square metres of usable roof, including space between rows and around the edges. Using modern panels of around 430 watts:

• 50kWp: about 115 panels, needing 250 to 300 square metres
• 100kWp: about 230 panels, needing 500 to 600 square metres
• 250kWp: about 580 panels, needing 1,250 to 1,500 square metres

Usable area is usually less than the total roof. Installers leave clearance from edges and ridges, avoid rooflights, vents, plant and parapet shading, and keep access routes for maintenance and fire safety. On many commercial roofs 60% to 80% of the gross area is usable.

Orientation changes the layout. Pitched south-facing roofs give the highest yield per panel. Flat roofs often use east-west layouts at a low tilt, which fit more panels into the same area at a slightly lower yield per kWp and spread generation across the day.

The roof must also carry the load. Ask for a structural assessment, especially on older buildings, lightweight portal frames and flat roofs needing ballasted frames.

The local electricity network can cap the size of system you can connect, or at least how much it can export. Systems up to 16 amps per phase, which is 3.68kW on a single-phase supply and 11.04kW on three-phase, connect under the simpler G98 notification process. Almost every commercial system is larger and needs approval from the distribution network operator (DNO) under G99 before it is switched on.

The DNO has up to 45 working days to issue a connection offer on a full application. Where the local network is constrained, it may offer the connection only with an export limit, or require reinforcement works at your cost.

Export limitation is often the practical answer. The inverters are set to stop the site exporting more than an agreed figure, which can be zero. Because a system sized to your demand exports relatively little anyway, an export limit usually costs very little in lost income. It can also let you install a larger system than the network would otherwise accept, with any surplus curtailed rather than exported.

Sizing to current demand is the default, but there are good reasons to go larger:

• Electrification plans: moving a vehicle fleet to electric, adding workplace EV charging or replacing gas heating with heat pumps will raise daytime demand. If those plans are firm and near-term, size for the future load.
• Battery storage: a battery can capture midday surplus for use later in the day, raising the self-consumption of a larger array.
• Business growth: new shifts, extra production lines or a planned extension.
• Neighbouring users: on multi-let estates, surplus can sometimes be supplied to tenants through a private wire arrangement.
• Better export prices: a strong export tariff or a commercial export agreement improves the value of surplus generation.

Fitting scaffolding, design and the grid application twice costs more than doing it once, so if growth is likely within a few years, a slightly larger system now can be good value. Ask for the case to be modelled both on today's demand and the expected future demand.

The inverters convert the panels' DC output to AC for your building, and their combined rating is often lower than the panel capacity. A ratio of panel kWp to inverter kW of around 1.1 to 1.3 is common in the UK, because panels rarely reach their full rated output in British conditions.

For example, a 100kWp array might use 80 to 90kW of inverter capacity. On the sunniest days the inverters cap output briefly, a small loss known as clipping, but the cost saving on inverters usually outweighs it. The inverter rating also matters for the grid application, because the DNO assesses the export capability of the equipment.

Commercial installers commonly use string inverters from manufacturers such as SMA, Huawei, Solis, Fronius and SolarEdge, and may add optimisers where parts of the roof are shaded or face different directions. Large systems may be split across several inverters, which improves resilience if one fails.

Ask each installer to state the DC to AC ratio and the expected clipping loss in their yield model, so you can compare designs on equal terms.

Every site is different and your own data comes first, but these patterns are a useful starting point when you review proposals from installers:

• Small office or shop: often 10 to 30kWp, limited by roof size and modest daytime demand.
• Large office: typically 30 to 100kWp. Strong weekday match, but weekend generation is largely exported unless storage is added.
• Light industrial unit: often 50 to 150kWp, depending on machinery and operating hours.
• Chilled or frozen distribution: 150kWp to 1MW or more, because refrigeration demand peaks with sunshine.
• Ambient warehouse: roof space often exceeds demand, so systems are commonly limited to 100 to 250kWp unless the space is let to a third party or a battery is added.
• Manufacturing plant: 250kWp to several MW on sites with high continuous load.
• School: 30 to 150kWp, sized with the long summer holiday in mind.

Explore sector options on our manufacturing, retail and warehousing and agriculture pages.

Gather this information before approaching installers, and you will get designs that can be compared fairly, line by line, rather than three proposals built on different guesses:

1. Twelve months of half-hourly consumption data, or at least 12 monthly bills
2. Your current unit rate and contract end date
3. Normal operating hours, weekend use and shutdown periods
4. Any firm plans for EV charging, heat pumps, new equipment or expansion
5. Roof drawings, building age and construction, if available
6. Known roof issues, such as leaks, fragile sheets or asbestos
7. Your supply capacity and whether it is single-phase or three-phase

When proposals arrive, check the self-consumption percentage, the marginal payback of each size option and whether an export limit has been assumed. Our guides to commercial solar payback, comparing quotes and commercial solar costs cover each step in more detail. When you are ready, compare quotes from up to three MCS-certified commercial installers.