How many solar panels fit on my roof? A simple way to work it out

It is the first question almost everyone asks, and the honest answer is "fewer than the roof area suggests". Edges, vents, chimneys, skylights and the shape of each roof face all eat into the space you can actually use. The good news is that you can get a reliable number in a few minutes once you know what to measure.

#Start with the size of a panel

Most rooftop panels sold today are close to the same size, even across brands:

So one residential panel takes up roughly 1.9 to 2 m² and produces around 400–440 W at peak. Always check the datasheet of the exact model you plan to use, because a few centimetres per panel adds up across a row.

#Measure the usable area, not the whole roof

Each roof face is its own little design. For every face:

1. Measure its real dimensions. Use the slope length, not the footprint seen from above. A 30° pitched roof is about 15% longer along the slope than it looks on a map.
2. Take off the edge clearance. Many places require a gap along ridges, eaves and edges for fire access and wind loads. Local rules vary a lot (from about 0.3 m to over 1 m), so check what applies to you. 0.5 m is a sensible planning figure.
3. Remove obstacles with a margin. Chimneys, vents, skylights, water tanks and satellite dishes need a clear zone around them, and anything that casts a shadow should push panels further away.
4. Ignore faces that point away from the sun. In the northern hemisphere a north-facing face usually produces too little to be worth it (and the reverse in the southern hemisphere).

#A worked example

Take a south-facing roof face that is 8 m wide and 5 m deep along the slope.

• With a 0.5 m clearance all round, the usable rectangle is 7 m × 4 m.
• Portrait panels (1.13 m wide, 1.72 m tall) fit 6 across (6 × 1.13 = 6.78 m, with gaps) and 2 rows high (2 × 1.72 = 3.44 m).
• That is 12 panels. At 430 W each, the face holds 5.16 kWp.

Try landscape too: panels on their side fit 4 across (4 × 1.72 = 6.88 m) and 3 rows (3 × 1.13 = 3.39 m), which is also 12. On a different roof shape one orientation often wins by a full row, so it is always worth checking both.

Rule of thumb: on a simple, unobstructed face you can usually cover 60–75% of the area with panels. Hips, valleys and lots of vents bring that down quickly.

#Why roof shape matters more than area

Two roofs with the same area can hold very different numbers of panels:

• Triangular and trapezoid faces (hip roofs) lose space at the slanted sides because panels are rectangles.
• Narrow faces may fit one row where the area suggests two.
• Dormers and skylights in the middle of a face split it into smaller pieces, each with its own edge clearance.

This is why a quick "area ÷ 2 m²" estimate usually overshoots by 20–40%.

#From panel count to system size

Once you know the panel count:

• System size (kWp) = number of panels × panel power ÷ 1000.
• Yearly generation depends on where you are and which way the roof faces. In much of India, 1 kWp typically produces around 1,300–1,600 kWh a year. In central Europe the figure is more like 900–1,100 kWh.

So the 5.16 kWp face in the example could produce roughly 7,000–8,000 kWh a year in a sunny climate. That is often enough to cover a family home's use.

#Do it on your own roof

Measuring each face by hand works, but it is slow and easy to get wrong on complicated roofs. In Solarqivo you trace each roof face on satellite imagery, set the edge clearance and mark the obstacles. The designer then fills every face with panels automatically. You can mix panel models, switch between portrait and landscape and see the yearly generation straight away.

If you want to understand what direction and tilt do to those numbers, read our guide to the best tilt and direction for solar panels.