Do you know if your PV system is performing as well as it
should? Do you know if your system is performing as well as other
local PV systems? Are you interested in comparing the performance
of your system with others in the UK.
Well it is relatively simple to answer all of these
When you had your system installed your installer had to give
you an estimate of what your PV system should generate in an
average year. This is known as a SAP calculation. The SAP
calculation calculates the output of a solar PV system according to
the Government's Standard Assessment Procedure for Energy Rating of
Dwellings (SAP 2005). This assessment takes into account such
factors as shading, orientation and angle of the solar panels. It
is usually quite conservative and don't take account the precise
whereabouts of your property.
So as a starting point you have the installers estimate to
compare your actual output.
However the PV output varies throughout the year due to the
height of the sun and the length of the days among other climatic
factors. It is very helpful to know what your system should be
generating on a monthly basis so that you can compare your actual
generation figures with a monthly estimate. Such monthly estimates
can be obtained from the PVGIS
website. Help in using the PVGIS calculator can be found on
So now you can compare your actual PV output figures with a
monthly estimate specific to your actual latitude and
Finally there are organisations who collate data from hundreds
to thousands of individual subscribers, enabling those subscribers
to compare their systems overall performance with other individuals
locally or in other parts of the world. One such websites is :-
Microgen database organised by the University of Sheffield,
also known as the Sheffield Solar Farm. This website offers a wide
range of comparisons, a forum to ask questions and a monthly
So now you can compare your actual output with other local PV
generators which will help you evaluate whether local weather
conditions for example are impacting your output.
The University of Sheffield have developed an application with enables you to compare the
annual generation predicted by SAP, PVGIS and the Sheffield
Solar Farm's database.
You will need your installation's location by longitude and
latitude. To find these figures from your postcode, you can use
public tool: http://www.doogal.co.uk/ShowMap.php?postcode=EX17
2AH but add your own postcode to the end.
So with all this potential data and comparison sites it should
be possible for you to evaluate whether your system is performing
properly. If it is not, then you should contact your installer.