How many solar panels do I need? The honest way to count

Most UK homes need 6 to 20 panels, and the honest way to land on your own number is geometry, not a national average. One modern panel has fixed dimensions. Your roof has a width you can measure from the pavement and a slope you can estimate from the pitch. Divide one by the other, take off the clearances and the obstacles, and you have your count before anyone rings you back. Below is that arithmetic done properly, worked examples for the three most common British house types, and the reasons the final number still belongs to a survey.

Start with the panel, not the roof

Panel counting only works if you fix the panel first, because panel sizes vary by brand and vintage. The panel we design around is the LONGi Hi-MO X10: 530 W, 1990 mm tall by 1134 mm wide, which is 2.26 square metres of roof per panel. That is a physically large module. In portrait, two rows stacked up the slope need just under 4 metres of rafter length. In landscape, a row is only 1134 mm high, which is how shallow faces above garages and bay windows earn a row that portrait panels would waste.

Every panel you fit adds 530 W to the array, so the count converts straight into system size: 10 panels is 5.3 kWp, 14 panels is 7.4 kWp. At the installed price we work to, £1,000 per kWp, that same count also converts straight into the solar side of the budget, so 10 panels lands around £5,300 installed. Getting the count right is the same job as getting the quote right.

Why panels stop short of the edges

Look at any well installed array and the panels finish a visible margin short of the ridge, the verge and the eaves. That margin exists because wind does not load a roof evenly. Air accelerating over the edges and corners of a pitched roof produces the strongest uplift exactly there, so the mounting design keeps the array out of the worst zones, and MCS installation standards require the mounting arrangement to be assessed for wind uplift on your specific roof, not copied from the last job. The margin also keeps the roof hooks in sound, full tile courses rather than cut edge tiles, and leaves access around the array for maintenance.

Measure your roof from the ground

Obstacles cost more than their own footprint

The clean rectangle almost never survives contact with a real roof. Each obstacle removes not just the tiles it sits on but every panel position it interrupts, because panels come in one size and do not flow around things.

• Roof windows. A Velux in the middle of a face is the most expensive obstacle per square metre, because it breaks both a row and a column. One window can turn a 10 panel face into an 8 panel face.
• Chimneys. A chimney costs its own footprint plus a working clearance, and then keeps costing after installation, because its shadow sweeps across the array through the day.
• Soil stacks and vents. Small in area, awkward in position. A vent pipe emerging mid slope can eliminate a full panel position on its own, though some can be rerouted or fitted with low profile terminals during the install.
• Aerials and satellite dishes. Usually the cheapest problem on the list, because they can often be relocated to a gable or the chimney before the panels go on.

Shading deserves its own line because it changes the value of positions rather than their existence. A position in the afternoon shadow of a chimney or a neighbouring dormer still physically holds a panel; whether it earns its place depends on the electronics. On string inverter systems one shaded panel drags down its whole string, while optimisers and microinverters let each panel contribute what it individually can, which is why shaded roofs push designs towards module level electronics. The trade offs sit in our inverter architecture guide.

Worked counts: terrace, semi, detached

Run the method above across the common British house types and consistent brackets emerge. They are illustrative arithmetic rather than survey data, and your own roof will land inside or near them unless it is unusual.

Brackets from applying the method above to typical UK house dimensions with the 530 W LONGi Hi-MO X10; kWp is the count multiplied by 0.53. Hipped roofs sit at the low end of each bracket because the sloping hip edges cut the rectangle into a trapezium.

The terrace bracket is worth unpacking because it looks low until you see how it is made. A terrace face 4.5 metres wide takes three portrait columns after clearances, and most terrace slopes take one portrait row plus, sometimes, a landscape row above it. Three or four panels on one face sounds underwhelming, but most terraces run east west, so the same count repeats on the back face and the system totals 6 to 8 panels charging across the whole day. Whether that split orientation suits you is the subject of our east west versus south facing guide.

Planning rules rarely change the count

In England, solar on a house roof is permitted development under the GPDO provided the panels project no more than 200 mm from the roof surface and finish no higher than the highest part of the roof, excluding any chimney. Standard on roof mounting systems meet both comfortably, so for most homes the planning system never touches the panel count. The exceptions are listed buildings and protected land such as conservation areas, where conditions tighten and wall mounted or visible installations may need consent. If that is you, raise it early rather than after the design.

Why the survey still decides

The arithmetic above gets you to within a panel or two, and that is exactly what it is for: sense checking quotes and setting expectations before anyone sells you anything. What it cannot see is what sits under and around the tiles. Rafter condition and spacing, the true pitch rather than the estimated one, the wind exposure of your particular street, whether that soil stack can move, and the inverter and export limits your network operator will accept all belong to the design stage, and any of them can move the final count.

This is also why the panel count and the system size are settled together rather than in sequence. A roof that holds 14 panels does not automatically want 14; your consumption, battery plans and export arrangements might land the right answer at 10. That ordering is the subject of our system sizing guide, and the budget side lives in what drives solar cost in 2026.