Commercial Solar System Sizes: From 10 kW to 1 MW

How system size is measured: kW and kWp

kWp (kilowatt peak) is the panels' combined output under standard test conditions. Proposals often write it as kW. Two other figures matter as much: the inverter rating, which can be lower than the panels' kWp and sets what the network operator is asked to allow, and the expected yearly generation in kWh, which is what saves you money.

Yearly generation is roughly the kWp times the yield for the site. The PVGIS yields in our payback calculator run from about 810 kWh per kWp in the Highlands and North Wales to about 1,160 on the south coast, with Birmingham at 968 and Exeter at 1,052 (PVGIS 5.3). The roof's direction, tilt and shading change it again, which is what the survey checks.

Simple multiplication, for a sense of scale only. Your site's figure comes from the installer's design.

10 to 30 kW: shops, offices and small units

The size for a shop, a small office, a workshop, a surgery or a single industrial unit. DESNZ's 2025/26 averages for non-domestic systems are £1,320 per kW for 4 to 10 kW and £1,167 per kW for 10 to 50 kW (source), so a 20 kW system works out at about 20 × £1,167 = £23,340 as an average, and a 30 kW one at £35,010.

Most systems of this size are above the G98 limit of 16A per phase, so the network operator is asked before connection under G99 (NGED). Our worked example of a 30 kWp office in Birmingham is in the payback guide.

50 kW: what it takes and who it suits

50 kW is a common size for a farm building, a mid-sized factory unit, a hotel or a school. It is also a line in the rules: MCS's solar PV installation standard covers systems up to a maximum DC output of 50 kWp (MIS 3002), and DESNZ's cost data stops at the same point. At the 10 to 50 kW average, 50 kW is 50 × £1,167 = £58,350.

Our 50 kWp farm example near Exeter generates 52,610 kWh a year and pays back in 7.6 years on its stated assumptions; see the payback guide.

100 to 250 kW: factories, warehouses and farms

Large roofs on warehouses, factories and farm buildings can take 100 kW or more. At this size there is no government cost average, and the design depends on things a calculator can't see:

• The roof's structure. Whether it can carry the extra load, and its condition and remaining life.
• The connection. Whether the network can take the export, or whether the system needs to limit what it exports.
• Your daytime use. A system this size can easily make more than the site uses on a quiet day.
• Certification. Above 50 kWp the work is outside MIS 3002. Ask the installer how the system will be designed, tested and certified.

See warehouses, factories and farms for what the survey checks on each.

500 kW to 1 MW: large sites, the grid and planning

At this scale the grid connection usually decides the timetable and the design. The Smart Export Guarantee still applies: Ofgem says it covers installations up to 5 MW (Ofgem).

On planning, Class J of the permitted development order in England sets no capacity limit for solar on the roof of a non-domestic building (GPDO Part 14). Its conditions are about position: on a pitched roof, panels can't protrude more than 0.2 metres beyond the roof slope; on a flat roof, no higher than 1 metre above its highest part; nothing within 1 metre of the roof's edge; and not on a listed building or scheduled monument. Ground-mounted arrays and carports have their own rules: see solar carports.

What changes the price per kW

• Size. DESNZ's data shows the cost per kW falling as systems get bigger.
• The roof. Fixings for metal, fibre cement or flat roofs, ballast, and any repairs first.
• Access. Scaffolding, edge protection, fragile rooflights and keeping the business open.
• The connection. Network studies, export limitation or upgrades on larger systems.
• Equipment and warranties. The panels, inverters, monitoring and any battery.