How Much Solar PV Do I Need?

Building 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

For community buildings it is advisable to check with your local planning office to ascertain what changes you can make under permitted development rights to ensure you will not be penalised for placing PV panels on the roof.

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, 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 building;

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 to work out the length of your pitched roof using measurements of A and B at the gable end (see picture below):

A2 + B2 = C2

and then:

x Roof Width = Roof Area (One side of a pitched roof on your building)

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

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 £8,000 - £10,000 for an average sized system to be installed on an average roof. Obviously, the larger the community building the greater your demand for panels, and the more panels you place on your building the greater the cost.

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. You should be able to work out your demand by studying your quarterly energy bills.

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