Two numbers decide whether a system fits your roof. How many panels you need, and how much room each panel takes.
This is the reference page for both. Panel sizes come from the manufacturers' own datasheets, and the tables cover every system size from 3 kWp to 10 kWp. For the step by step version, read how many solar panels do I need.
Key Points
- We fit 450 W to 500 W panels on homes. Commercial roofs take larger panels.
- One domestic panel covers about 2.0 to 2.2 square metres, frame included.
- A 4 kWp system is 8 or 9 panels and about 16 to 19 square metres of panel.
- Your usable roof is smaller than your roof. Edges, the ridge, the gutter and vents all take space out.
- The 3.68 kW per phase figure limits connected generation capacity, not panel count.
The panels on our published work
| Panel | Rated power on our jobs | Module size | Area | Cells |
|---|---|---|---|---|
| Aiko Neostar 3S+54 | 475 W | 1762 x 1134 x 30 mm | about 2.00 m2 | 108 |
| Jinko Tiger Neo 60HL4 | 470 W | 1903 x 1134 x 30 mm | about 2.16 m2 | 120 |
| JA Solar JAM54D41 (LB variant) | 460 W | 1762 x 1134 x 30 mm | about 2.00 m2 | 108 |
Two honest notes. The Aiko datasheet we opened covers the dual glass version of the same Neostar 3S+54 family, so that frame size is the family figure. The JA Solar row is the LB variant, which runs 430 W to 455 W; no datasheet for the exact 460 W variant on our jobs would open.
Common Misconceptions
"A 4 kW system is 10 panels." That came from 400 W panels. At the panels we fit it is 8 or 9.
"If my roof is 30 square metres I can fit 30 square metres of panel." No. Take off the edges, the ridge, the gutter line and anything sticking through.
"3.68 kW is a limit on my panels." No. It limits connected generation capacity per phase, which is normally the inverter's AC output. A bigger panel is also not always better: a larger module may not fit the width of a small roof face at all.
Real-World Advice
Our published 13 panel installation in Hastings is a 6.18 kWp array of Aiko Energy AIKO-A475-MCE54Mb panels. See the Hastings case study.
| Step | Figure | Where it comes from |
|---|---|---|
| Rated power per panel | 475 W | Aiko Neostar 3S+54 datasheet |
| System size | 6.18 kWp | 13 multiplied by 475 W |
| Area of one panel | about 2.00 m2 | 1.762 m by 1.134 m from the datasheet |
| Panel area on the roof | about 26 m2 | 13 multiplied by 2.00 |
| Modelled output facing south | about 7,100 kWh a year | 6.18 multiplied by the PVGIS figure below |
So 13 panels take about 26 square metres, on a roof comfortably bigger than that.
UK-Specific Considerations
Panels per system size
System size in kWp is all the panels' rated power added together. Divide by 500 W and by 450 W to get the range.
| System size | Panels at 450 W to 500 W | Panel area | Modelled output facing south |
|---|---|---|---|
| 3 kWp | 6 to 7 | about 12 to 15 m2 | about 3,500 kWh a year |
| 4 kWp | 8 to 9 | about 16 to 19 m2 | about 4,600 kWh a year |
| 5 kWp | 10 to 12 | about 20 to 26 m2 | about 5,800 kWh a year |
| 6 kWp | 12 to 14 | about 24 to 30 m2 | about 6,900 kWh a year |
| 7 kWp | 14 to 16 | about 28 to 35 m2 | about 8,100 kWh a year |
| 8 kWp | 16 to 18 | about 32 to 39 m2 | about 9,300 kWh a year |
| 9 kWp | 18 to 20 | about 36 to 43 m2 | about 10,400 kWh a year |
| 10 kWp | 20 to 23 | about 40 to 50 m2 | about 11,600 kWh a year |
The area column uses 2.0 square metres at the low end and 2.16 at the high end. The output column is modelled, not measured: we ran the European Commission's PVGIS tool for Eastbourne at a 35 degree pitch with 14% losses. It gives 1,156 kWh a year per kWp facing south, 915 kWh facing west and 879 kWh facing east. An east or west roof does about three quarters of what a south roof does, and shade cuts it again. See panel orientation best practices.
Usable roof is smaller than roof
| What takes space | Why |
|---|---|
| The verge and the edges | Panels are held in so wind cannot get under the array |
| The ridge | Panels are kept clear of the ridge line. The planning rule is separate, and only says no part may sit higher than the highest part of the roof |
| The gutter line | The bottom row is held clear of the gutter and eaves |
| Vents, soil pipes and flues | Nothing mounts over them, and the gap around one wastes more than the pipe |
| Dormers, hips, valleys, roof windows | They break the roof into faces too small for a full row |
Those clearances are what we set at survey. They are not one published number, because they depend on the roof covering, the fixing system and how exposed the site is. Ask for the layout drawing rather than a panel count.
Planning sets two limits in law. Equipment on a house must not stick out more than 0.2 metres beyond the plane of the roof slope, and no part may be higher than the highest part of the roof, not counting the chimney. On a flat roof the limit is 0.6 metres above the highest part, and listed buildings are excluded. See roof suitability by roof type.
The 3.68 kW per phase line
Engineering Recommendation G98 covers the connect and notify route. It applies where the aggregate Registered Capacity is no greater than 16 A per phase, which G98 states is 3.68 kW on a single phase supply and 11.04 kW on a three phase supply.
Registered Capacity is not the panels' kWp rating. G98 defines it as the designed maximum active power capacity of the generator, and says that where the generator connects through an inverter it is the lesser of the inverter rating or the rating of the energy source. On a home system that is the inverter's AC output.
So a 4 kWp array on a 3.68 kW inverter still sits inside the G98 route. Above 16 A per phase the G99 process applies and the network operator must be consulted first. We often fit larger inverters than that, and we make the G99 application as part of the job. On the G98 route the installer sends the details no later than 28 days after commissioning. See the G98 application process.
When This Isn't Right For You
Roof area alone does not tell you a roof is suitable. A large north facing roof still generates far less than a small south one, and a roof in poor condition should be repaired before anything is fixed to it.
This page does not cover flat roofs. Panels there sit on frames at a pitch and need spacing between rows, so the area per kWp is much larger.
Before You Spend Money
Ask for the exact panel model and its module dimensions from the datasheet, not a rounded "about two metres".
Ask for a layout drawing showing every panel with the clearances marked, and ask what was left off it and why.
Ask what the array's kWp is and, separately, what the inverter's AC output is. Only one of those decides your connection route. Ask which roof faces the panels go on, and the design output for each.
Frequently Asked Questions
Q: How much roof space does a 4 kWp system need?
A: About 16 to 19 square metres of panel, on a noticeably bigger roof than that. It is 8 or 9 panels at the 450 W to 500 W sizes we fit.
Q: How big is one solar panel?
A: The panels on our published work are 1762 mm by 1134 mm for the Aiko and JA Solar modules and 1903 mm by 1134 mm for the Jinko. That is about 2.0 to 2.2 square metres each.
Q: How much will each kWp generate?
A: PVGIS models a roof in Eastbourne at a 35 degree pitch with 14% losses at 1,156 kWh a year per kWp facing south, 915 kWh facing west and 879 kWh facing east. Those are model figures for a clear roof.
Q: Does the 3.68 kW limit cap how many panels I can have?
A: No. It caps connected generation capacity per phase, normally the inverter's AC output. A larger array behind a smaller inverter can still use the G98 route.
Summary
Allow about 2.0 to 2.2 square metres per panel. A 4 kWp system is 8 or 9 panels and about 16 to 19 square metres; a 10 kWp system is 20 to 23 panels and about 40 to 50 square metres. Modelled output in Eastbourne is 1,156 kWh a year per kWp facing south, and roughly three quarters of that facing east or west. Your usable roof is always smaller than your roof, and 3.68 kW per phase is about connected generation capacity, not panels. For a real layout, see solar installation.