How many solar panels can I fit on my roof?

UPDATE: The Feed-In Tariff is now closed for new applications. To find out about the new scheme designed to replace it, click here.

Solar panels are now an affordable way to reduce your demand on the electric grid by producing your own electricity. That is without even taking into account the Feed-in Tariff, which pays you for every unit of electricity created, regardless of whether you use it in the home or sell it back to the grid.

How big a system?

With a 3.5kw system now costing about £6,500 and annual returns of £800 plus, getting solar PV panels installed really is an attractive investment opportunity.

The key to making the numbers work though is maximising the number of panels you can squeeze onto your roof, because regardless of the size of system, you are still going to need things like inverters and cabling to run through the home, so the actual cost of adding an extra panel is not really going to add that much to the final cost of the system.

Most solar panels are 250 watts; therefore to get a 3.5kW (or 3500 watts) system you would need 14 panels.

250 watt solar PV panels are all pretty much a standardised size – they are around 1.6m x 0.9m and about 5cm thick.

For example the Suntech 250w monocrystalline panel is 1665mm x 991mm x 50mm, whhereas the Sanyo/Panasonic panel is 1610mm x 861mm x 35mm, but broadly speaking they are all in that range. Therefore when sizing your solar PV system we recommend using this to drive your calculations.

With a panel therefore being approximately 1.44m2 in total, to get 14 panels on a roof you need a space of about 20m2. However roof-mounted solar installations must also be more than 30cm away from the external edge of the roof, meaning that actually you will need an even larger space so this needs to be factored in to any roof sizing calculations.

Shading and orientation

Your panels will need to be positioned on a South (or South-East to South-West) facing roof. The further away your panels are from a southerly orientation, the less effective they will be, although 45 degrees either way will still produce more than 90% of the energy of a due south oriented system.

Another thing to take into account is shading – essentially if you have a chimney that casts a shadow across the roof at certain times of the day, this will severely diminish the return from the solar system. In this case you should try to position the solar panels out of this shading to maximise the amount of electricity you can create – again this might diminish the amount of roof space you have.

>>> Maximise your solar return! <<<

In terms of calculating the actual roof space that you have, it is relatively simple to do this if you have access to the loft space. Essentially you need to take 3 measurements to do this.

  1. Measure the height from the highest point of the roof, vertically down to the joists (assuming this is the apex).
  2. Measure the distance from the apex horizontally to the eaves of the roof at joist level.
  3. Measure the length of the apex of the roof.

With these 3 numbers, you can use a bit of GCSE maths to calculate the area of your roof.


A2 + B2 = X2


Once you calculate X2, you need to square root it – to give you X.

X gives you the distance from the apex down to the edge of the roof.



Multiplying C by X will give the area of roof space available. You also need to deduct the 30cm around the edge of the roof on which the panels cannot be fitted – this area will depend on the type of property – detached, terraced etc.

Flat roofs

If you have a flat roof, the panels will usually be mounted at an angle to maximise efficiency. While panels can be situated right next to each in a row, care must be taken to position them in a place where they don’t cast shadows on the row behind or in front.

Size of system versus roof space

Size of system No. of panels Panel area (m2) Roof area required (m2)
2.50kW 10 14.4 19.4
2.75kW 11 15.8 21.2
3.00kW 12 17.3 22.9
3.25kW 13 18.7 24.6
3.50kW 14 20.2 26.4
3.75kW 15 21.6 28.1
4.00kW 16 23.0 29.8

Installing Solar PV

Are you thinking about installing a solar PV system at home? We have scoured the country for the best tradespeople, so that we can make sure we only recommend those we really trust.

If you would like us to find you a local installer to help install a solar PV system in your home, just fill in the form below and we will be in touch shortly!

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