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.
Approximate yearly generation by system size, from PVGIS regional yields
| System size | At 810 kWh per kWp | At 1,160 kWh per kWp |
|---|---|---|
| 10 kWp | 8,100 kWh | 11,600 kWh |
| 20 kWp | 16,200 kWh | 23,200 kWh |
| 30 kWp | 24,300 kWh | 34,800 kWh |
| 50 kWp | 40,500 kWh | 58,000 kWh |
| 100 kWp | 81,000 kWh | 116,000 kWh |
| 250 kWp | 202,500 kWh | 290,000 kWh |
| 1 MWp | 810,000 kWh | 1,160,000 kWh |
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.
Sources
Costs, yields and thresholds were checked on 2 November 2026 (the PVGIS yields were taken for our payback calculator on 23 October 2026). Check the source before relying on a figure.
- DESNZ: Solar PV cost data 2025/26 (published 28 May 2026)
- European Commission JRC: PVGIS 5.3 (regional yields used in our calculator)
- National Grid Electricity Distribution: G98 micro-generation (under 16A per phase, notify within 28 days)
- National Grid Electricity Distribution: G99 generation (over 16A, 3.68 kW, per phase)
- MCS: MIS 3002:2025 Solar PV Installation Standard, version 2.0 (up to 50 kWp DC)
- Ofgem: Smart Export Guarantee (installations up to 5 MW)
- legislation.gov.uk: GPDO 2015, Schedule 2, Part 14 (Class J, solar on non-domestic buildings)
Frequently asked questions
How much does a 50 kW commercial solar system cost?
The government's 2025/26 average for non-domestic systems of 10 to 50 kW is £1,167 per kW, which gives 50 × £1,167 = £58,350 as an average. That is the figure in our worked example, not a quote: your price depends on the roof, access, equipment and connection.
How much electricity does a 50 kW solar system produce?
Roughly 50 times the yield per kWp for the site. PVGIS gives about 810 to 1,160 kWh per kWp a year across the UK regions in our calculator, so about 40,000 to 58,000 kWh a year before the roof's direction, tilt and shading are taken into account. Our 50 kWp example near Exeter makes 52,610 kWh a year.
How much does a 100 kW solar system cost?
There is no government average above 50 kW. DESNZ's data stops there because it comes from MCS installations, and it shows the cost per kW falling as systems get bigger. A 100 kW price comes from a quote after a survey.
Does a 30 kW system need permission from the network operator?
In most cases, yes, before it connects. Under 16A per phase a system can be connected and the network operator told within 28 days (G98). Above 16A, which is 3.68 kW, per phase, G99 applies and the installer applies to the network operator first.
Is there a size limit for solar on a business roof without planning permission?
In England, Class J of the permitted development order has no capacity limit for solar on a non-domestic building's roof, but it has other conditions: panels on a pitched roof can't stick out more than 0.2 metres, on a flat roof no more than 1 metre above the highest part of the roof, and none within 1 metre of the roof's edge. Listed buildings are excluded.
What size solar system does my business need?
Size it to how much electricity you use in daylight, not to fill the roof. The installer works it out from your bills or half-hourly data at the survey.
Related pages
- Sizing your systemHow to size commercial solar from consumption data, roof area and grid limits, with typical rules of thumb, a worked illustrative example and notes on batteries and future loads.
Commercial solar costIllustrative prices per kWp by system size, extra costs, running costs, VAT and tax.- G99 and DNO applicationsG98 versus G99, what goes into a DNO application, connection offers, export limitation under G100, reinforcement costs and how the survey checks your supply.
- Small businessesSmaller systems on shops, industrial units and workshops can pay back where the business uses power in the day; the lease and supply matter.