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
3040 degrees; if your property has a flat roof, aframes 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 southfacing 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 SouthWest 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 
SemiDetached 
ВЈ805 
ВЈ619 
ВЈ666 
ВЈ796 
ВЈ1,059 
ВЈ886 
Detached 
ВЈ924 
ВЈ300 
ВЈ917 
ВЈ797 
ВЈ991 
ВЈ1,038 

Taking a 4 bedroom semidetached 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.