A 50 kW Solar System: Output, Roof Area and the 50 kW Thresholds
Updated 23 October 2026 · SEO Dons Editorial
Why is 50 kW the line for smaller commercial systems?
Several rules draw a line at 50 kW, and they are not all measured the same way. The table sets out what each source says.
| Rule | What it says about 50 kW | Source |
|---|---|---|
| Microgeneration | Electricity plant counts as microgeneration if its capacity to generate does not exceed 50 kilowatts | Energy Act 2004, section 82 |
| Planning in England | A roof array that is microgeneration falls in Class J(b). Any other roof array falls in Class J(c), which needs a prior-approval determination first, with glare the named concern | GPDO 2015, Schedule 2, Part 14, Class J |
| Grid application | NGED describes a simplified application for aggregate capacity of 50 kW or less on three phases, and a standard application above that | NGED G99 page |
| Export meter reference | SSEN says the supplier arranges the export MPAN under 30 kW, its network planner between 30 and 50 kW, and an account manager above 50 kW | SSEN generation connections page |
| Capacity registers | The Embedded Capacity Register lists generation and storage of 50 kW or more that is connected or accepted to connect | NGED and ENWL register pages |
| Smart Export Guarantee | Not a 50 kW line: solar PV up to 5 MW of total installed capacity is eligible | Ofgem |
The 3.68 kW per phase line (11.04 kW across three phases) sits below all of these. The Energy Networks Association (ENA) says a generator below that capacity falls under G98 and one above it under G99, so a 50 kW system is always G99. Suppliers set their own evidence rules for export payment: British Gas, for example, lists a "Microgeneration Certification Scheme (MCS) certificate, Flexi-Orb certificate or accreditation certificate" as acceptable evidence. The Smart Export Guarantee guide covers export payment, and certification is on the MCS page.
Is the 50 kW line measured in kWp or kW?
Section 82 refers to the "capacity" of plant "to generate electricity" and does not say whether that means the DC peak of the panels or the AC output of the inverter. NGED, for its connection process, treats solar PV as having the size of the inverter's continuous steady state output.
The difference bites near the line. With 550 W modules, 90 panels make 49.5 kWp and 91 make 50.05 kWp. At 450 W the figures are 111 panels for 49.95 kWp, and at 600 W they are 83 panels for 49.8 kWp. A design with more panel kWp than inverter kW sits on different sides of the line depending on which figure a rule uses (the kWp, kW and kWh page explains why panels and inverters differ). An inverter limited to 50 kW can serve more than 50 kWp of panels, and whether such a design counts as 50 kW is exactly the question above. Ask the planning authority and the network operator, in writing, which figure they apply to the design in front of you.
How many panels and how much roof for 30 to 50 kWp?
Panel count is capacity divided by module wattage, rounded up. The table shows the count needed to reach each target and the capacity that results. To stay at or below 50 kWp exactly, the counts are 111 (450 W), 90 (550 W) and 83 (600 W).
| Target | 450 W modules | 550 W modules | 600 W modules |
|---|---|---|---|
| 30 kWp | 67 (30.15 kWp) | 55 (30.25 kWp) | 50 (30.00 kWp) |
| 40 kWp | 89 (40.05 kWp) | 73 (40.15 kWp) | 67 (40.20 kWp) |
| 50 kWp | 112 (50.40 kWp) | 91 (50.05 kWp) | 84 (50.40 kWp) |
Roof area is arithmetic, and these figures are illustrative. They assume a 550 W module of 2.28 m by 1.13 m (2.58 m²), so 91 modules have 234 m² of module surface. The assumed coverage is 95% for modules lying in the plane of a pitched slope and 90% for east-west pairs on a flat roof, which leaves room for small service gaps.
| Size | Module surface | Pitched roof, modules flush to the slope | Flat roof, east-west rows at 10° |
|---|---|---|---|
| 30 kWp (55 modules of 550 W) | 142 m² | 149 m² | 157 m² |
| 40 kWp (73 modules of 550 W) | 188 m² | 198 m² | 209 m² |
| 50 kWp (91 modules of 550 W) | 234 m² | 247 m² | 261 m² |
Flush mounting keeps modules within the 0.2 m Class J limit above a pitched slope, and to stay within Class J every module must be at least 1 m from the roof edge, so a usable area smaller than the building footprint is normal. Tilted south-facing racks on a flat roof need far more space because each row must clear the shadow of the one in front; the arithmetic is on the 100 kW page. Layout and wattage choices are part of the wider specification in the commercial PV systems guide.
How much does a 30 to 50 kWp system generate?
Annual output is capacity multiplied by the PVGIS yield for the place and layout, using 14% losses. Figures are rounded to the nearest 100 kWh.
| Location | kWh per kWp, south 35° | kWh per kWp, east-west 10° | 30 kWp, south 35° | 50 kWp, south 35° | 30 kWp, east-west | 50 kWp, east-west |
|---|---|---|---|---|---|---|
| Plymouth | 1,122 | 932 | 33,700 | 56,100 | 28,000 | 46,600 |
| Cardiff | 1,042 | 865 | 31,300 | 52,100 | 26,000 | 43,300 |
| London | 1,019 | 840 | 30,600 | 50,900 | 25,200 | 42,000 |
| Newcastle | 948 | 764 | 28,400 | 47,400 | 22,900 | 38,200 |
| Belfast | 919 | 756 | 27,600 | 45,900 | 22,700 | 37,800 |
| Glasgow | 853 | 707 | 25,600 | 42,600 | 21,200 | 35,300 |
Across these six places a 50 kWp array makes between about 35,300 kWh (east-west in Glasgow) and 56,100 kWh (south-facing 35 degrees in Plymouth) a year. The yield-by-region guide has 13 locations, and the savings calculator turns generation into a bill estimate.
What grid route does a 50 kW system follow?
A system at this size connects on three phases through a G99 route. NGED describes generation of "50kW 3 phase" or less as a smaller installation with a simplified application process. SSEN calls installations above 3.68 kW per phase but 50 kW or less "large scale microgeneration", notes that on a three-phase supply "each phase must be 17kW or less", and says it will need to agree with the customer how best to connect the generator, which can involve network modifications.
A simplified process does not settle the outcome: whether the local network can take the output still has to be assessed. Before sizing the inverter and any export limit, screen the site against the operator's capacity map (the heat maps guide lists where each one is). This page does not explain how to apply; the application is prepared by the installer.
Planning permission in England usually turns on Class J. A listed building, a building within the curtilage of one, and a scheduled monument site are excluded. That is the English order; Scotland, Wales and Northern Ireland need separate checks. You can send an enquiry through the site's form for a contractor to check a roof against these points.
Sources
- Energy Act 2004, section 82, legislation.gov.uk, legislation.gov.uk, checked 22 October 2026.
- Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 14, Class J, legislation.gov.uk, legislation.gov.uk, checked 22 October 2026.
- Connecting commercial generation to the electricity networks, Energy Networks Association, energynetworks.org, checked 22 October 2026.
- G99 connection procedures, National Grid Electricity Distribution, connections.nationalgrid.co.uk, checked 22 October 2026.
- Embedded capacity register, National Grid Electricity Distribution, commercial.nationalgrid.co.uk, checked 22 October 2026.
- Embedded capacity register, SP Electricity North West, enwl.co.uk, checked 22 October 2026.
- Large scale microgeneration (installations above 3.68kW per phase, but 50kW or less), Scottish and Southern Electricity Networks, ssen.co.uk, checked 22 October 2026.
- Smart Export Guarantee (SEG) - Generators, Ofgem, ofgem.gov.uk, checked 22 October 2026.
- Smart Export Guarantee tariff for businesses, British Gas, britishgas.co.uk, checked 22 October 2026.
- PVGIS 5.3 PVcalc runs (PVGIS-SARAH3, crystalline silicon, 1 kWp, 14% losses; south at 35 degrees, east and west at 10 degrees), European Commission Joint Research Centre, checked 22 October 2026: Plymouth south, east, west; Cardiff south, east, west; London south, east, west; Newcastle south, east, west; Belfast south, east, west; Glasgow south, east, west; and the PVGIS user manual.
Common questions
Is a 50 kW solar system the same as 50 kWp?
Not necessarily. 50 kWp describes the panels' DC capacity under test conditions, while 50 kW usually describes the inverter's AC output. NGED sizes a connection from the inverter and the Energy Act refers only to capacity, so confirm which figure each rule uses.
Does a 50 kW rooftop system need planning permission in England?
A roof array that counts as microgeneration (50 kilowatts or less) is permitted development under Class J(b), within limits on height above the roof and distance from its edge. Listed buildings, buildings in a listed building's curtilage and scheduled monument sites are excluded. Local conditions can differ, so confirm with the planning authority.
How many solar panels make 50 kW?
With 550 W modules, 90 panels make 49.5 kWp and 91 make 50.05 kWp. With 450 W modules it is 111 panels for 49.95 kWp, and with 600 W modules 83 panels for 49.8 kWp. Check the datasheet for the module quoted.
Would a 50 kW system appear on a network operator register?
The Embedded Capacity Register published by network operators lists generation and storage of 50 kW or more that is connected or accepted to connect. A system just under 50 kW would not normally be listed, while one at or above the line would.