How Much Solar PV Do I Need?

Home Suitability

There are three key areas to think about when you are considering the installation of a solar PV system on your roof:

1. Do you need planning permission?

2. Is your roof (or wall) in a suitable position?

3. Can the structure on which you are mounting the panels take the weight of the system?

1. Planning Permission

If your property is in England or Scotland you don't need planning permission to install a solar PV system as long as the system does not protrude more than 200mm above your roof line when installed. However, if your home is a listed building, lies in a conservation area or World Heritage Site you must contact your local planning office to check what you are able to do under permitted development rights.

Legislation regarding permitted development rights for the installation of renewable technologies in Wales and Northern Ireland is still yet to be passed; therefore you still need to get planning permission before installing a solar PV system. This is currently up for review with the Welsh and Northern Irish governments. Until a decision is reached please contact your local planning office to discuss any installations you wish to proceed with.

2. Roof Suitability

Ideally, a solar PV system should be installed on a south facing roof which has approx 10 to 20 square metres of roof space which is not obstructed by chimneys, dormers or shading from surrounding buildings and trees. Panels can be installed on pitched or flat roofs, although tiles can only go on pitched roofs. The most appropriate roof angle on which to install PV panels on in the UK is between 30-40 degrees; if your property has a flat roof, a-frames can be installed on which to mount the panels to hit the required pitch.

While it is recommended that solar PV works most efficiently on south facing roofs, you can install a system on a roof that is east or west facing and still attain good results. It is not recommended to install solar PV systems on north facing roofs in the UK as the output will drop to just over half of what it would be on a south-facing roof, and the amount of time required for the system to pay back its cost would increase dramatically.

3. Structural Considerations

Before purchasing a solar PV system it is important to make sure that the existing structure has sufficient load bearing capacity to support a system. For most roofs, particularly modern houses, this will not be a problem. If you are unsure about this consult with a buildings surveyor.

Sizing Guides

In the main, there are three different ways to size a PV system for your property;

1. The amount of panels you would like to put down in m2
2. Amount of kWp you wish to generate
3. The amount of money you have available to spend.

Kilowatt peak (kWp) is a measure of power output which stands for peak power. This value specifies the output power achieved by a solar panel under full solar radiation (1,000 watts per square metre).

Reasons for choosing one of these three approaches when considering a system will most likely either be budgetary, or concern the compliance with a specification for a project.

Whichever method you choose, one of the first things you need to do is identify the total area of the roof space you are placing your panels on, as this will immediately identify any problems you may encounter with what you want to do. E.g. it may transpire that you do not have enough south facing roof space to put down the number of panels you want or reach the desired kWp; or similarly, you may have additional room and could potentially put down more panels than originally expected and take further advantage of the governments feed in tariffs.

Calculating roof area can be done by using this simple formula:

A2 + B2 = C2

and then

x Roof Width = Roof Area (One side of the roof of your house)

*The above symbol is for square root, meaning you divide the value for C by itself.

Using the above method would give you the area of the roof section outlined in red on the picture below; you would already need to know the measurements for sections A and B on the gable end in order to carry this out.

If we assume that the house in this picture is orientated South South-West then the roof sections outlined in yellow and blue would also be suitable for solar panels. To calculate the area of the blue outlined roof space, simply carry out the same formula which was done for the red area; again you will need to know the measurements for A and B on the gable end.

To calculate the area of the yellow outlined roof space, work out the following:

A x B /2 = Roof Area

Sticking with our house in the picture, we now have the area for the 3 coloured sections of roof. However, the area of roof where the protruding section of house joins the red outlined roof space needs to be calculated and subtracted to give us an accurate total area, please see the diagram below:


Above is a plan view of the house in figure 1's roof, the area we need to calculate is the blue triangles. We already know the measurements for A and B as we used it to calculate the blue roof area, so all we need to do is:

A x B = Area (There is no need to divide this by two as A x B gives us the area of the square made up of two identically sized triangles such as the two side by side on the diagram above.)

Now, we subtract the above figure from the total area of the red roof, before adding on the areas for the blue and yellow roofs and that is how much area we have available for the placing of solar panels.

Once you have calculated your available roof area you will be able to see whether what you had in mind is feasible or just identify how much of the space you are going to fill with PV.


Typical Usage and Costs

It is very rare that two retrofitted PV systems are the same, and therefore when it comes to estimating costs which you as a customer can expect to incur, without conducting a site survey and studying plans in detail only a guide price can be provided. We would estimate a cost of ВЈ8,000 - ВЈ10,000 for an average sized system to be installed on an average roof.

Obviously, cost is very important, if not the most important, factor to consider when choosing a PV system; with payback seemingly being the key issue.

Economic payback is the length of time it takes for the system to recoup the money that you have spent on it. With PV this recovery will come in the form of reduced electricity bills from your supplier and the government run Feed in Tariff's. In order for you to be able to work out your payback, you first need to work out your current annual energy usage and then fit a PV system that provides a percentage of that amount.

The below two tables give average estimates for energy use and cost in different residential building types (figures provided by Energywatch):

Number of Bedrooms Avg. Electricity Consumption (kWh)
1 2,500
2 4,000
3 5,500
4 6,000
5 or more Add 250 kWh per room
Type of Property Average Price 1 Bed 2 Bed 3 Bed 4 Bed 5 Bed
Flat ВЈ675 ВЈ570 ВЈ743 ВЈ823 ВЈ575 ВЈ693
Terraced ВЈ774 ВЈ579 ВЈ710 ВЈ755 ВЈ1,085 ВЈ1,150
Bungalow ВЈ804 ВЈ929 ВЈ836 ВЈ707 ВЈ1,031 ВЈ1,200
Semi-Detached ВЈ805 ВЈ619 ВЈ666 ВЈ796 ВЈ1,059 ВЈ886
Detached ВЈ924 ВЈ300 ВЈ917 ВЈ797 ВЈ991 ВЈ1,038

Taking a 4 bedroom semi-detached bungalow as an example, we can see that the average energy use would be approx 6,000kWh per year and the cost of this would be just over ВЈ1,000. SIG Sustainable Solutions would recommend installing panels to generate no more than 6,000kWh of electricity per year as the additional benefit of selling electricity back to the grid is outweighed by the cost of buying a unit from your supplier. We would therefore where possible recommend that you use all you generate.

If you were to put the average system on the roof of this four bedroom semi detached house, we estimate you would be generating up to 1,500kWh per year; and this would therefore save you approximately ВЈ750 per year on your energy bills.

Everyone's homes and energy requirements are different, and we would advise you to read the above carefully to draw conclusions for your own property and if you need assistance please call us on 0845 873 8612.