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When our consultant first visits your home he or she will carefully measure the size, orientation and pitch of your roof so that we can calculate an accurate output figure that is specific to your home. The Solar PV industry is regulated under the Microgeneration Certification Scheme (MCS) and every installer under this scheme is required to use the government approved standard SAP 2009 mechanism for calculating system output. Our consultant will run through this with you and will give you an accurate and honest assessment of the potential of your particular site.
The last 6 months have seen a flood of new companies enter the solar PV market, many from a double-glazing background. Unfortunately some of their system performance claims are highly dubious to say the least. If any company offers you a system and claims it will significantly outperform what we have forecast, we would advise that you look deeper into the basis of those claims.
If you have been promised a higher figure we would love to discuss that with you. We feel it is important that the public understand how these figures are calculated, and we are more than happy to take the time to explain.
The SAP 2009 calculation in all its ’simple’ glory...
0.8 x kwp x S x ZPV
Explanation:
0.8
This factor takes into account a 20% loss in efficiency of a typical system. We design our systems to significantly reduce this loss and this is one area where we can “beat the system”.
KWP (kilowatt peak)
This is the total size of the proposed system, calculated simply by multiplying the amount of panels by the individual power rating of those panels e.g. 12 panels of a 220w panel is 12 x 220w = 2640wp (or 2.64kwp).
S
S is the annual solar irradiation from the table below and takes into account the angle of the proposed array and its pitch. The annual irradiation figure is an average taken from measurements across the UK over the last decade. This is the figure that overzealous salesmen will try to exaggerate. By publishing the government’s figures here we hope you will be able to see through these exaggerations!
| Tilt of collector | Orientation of collector |
|
South |
SE/SW |
E/W |
NE/NW |
North |
| Horizontal |
|
|
933 |
|
|
| 30o |
1073 |
1027 |
913 |
785 |
730 |
| 45o |
1054 |
997 |
854 |
686 |
640 |
| 60o |
989 |
927 |
776 |
597 |
500 |
| Vertical |
746 |
705 |
582 |
440 |
371 |
ZPV
ZPV is the over shading factor taken from the table below. Shading is the enemy of a good solar PV system and will significantly reduce its potential output. Any trees, buildings, chimney stacks etc that will cast a shadow across the area where the panels will be, has to be taken into account.
| Over-shading | % of sky blocked
by obstacles | Overshading factor |
| Heavy |
>80% |
0.5 |
| Significant |
>60% - 80% |
0.65 |
| Modest |
20% - 60% |
0.8 |
| None or very little |
<20% |
1.0 |
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