- There are over 200 different makes of Solar PV panel on the market currently. Each manufacturer gives each panel a W (Watt) rating – i.e. the number of Watts a PANEL will produce under peak test conditions. Typical panels come in the following W sizes ranging from 170W to 250W, with the “weakest and cheapest” panels being 175W and the most powerful being 250W).
- To create a system – something we call an “array” – you simply decide how many panels you want / can afford / can fit on the roof. The weaker panels (170) are much cheaper than the strongest (250W).
- Some clients will be more limited by the amount of roof space, and thus will be focused on the highest power panel that will fit in a certain area. Others will be more concerned about cost and will therefore look towards the cheaper, less powerful panels.
- Let us assume you want the strongest array you can get. Therefore, we work out you can fit 10 x 250W panels on a roof (Cernunnos do this on the site survey by measuring the roof). This would give a Wp system size of 2500Wp – or 2.5kWp (Kilo Watt Peak). That is, under peak conditions, the system would produce 2,500 Kilo Watt Hours per annum.
- However, the UK weather does not allow for Peak conditions. Thus we have to adjust for this and we do this by estimating how much the system will produce. There is an industry standard method for estimating how much a system will actually produce and it is called SAP 2005 (SAP = Standard Assessment Procedure). It is a country wide standard, with the base case city being Sheffield. Thus, in places that get more sun than Sheffield (i.e. the South of UK) SAP underestimates a system performance, and vice versa for northern UK.
- SAP takes into account the azimuth of the system (i.e. which way it faces: South, East or West etc); the angle of the system (i.e. the angle of which it is on the roof – an ideal angle being between 30 and 45 degrees); and how much shading there is (shading can affect a systems performance significantly). In general, SAP estimates that the actual output for a system will be approximately 86% of its kWp size. Thus a 1kWp system will produce 860kWh of electricity in any year in the UK.
Here is a Solar Radiation map of the UK. Anything South of Sheffield will outperform SAP and anything North will underperform.
The amount of energy a system will produce in each month of the year. Even though systems perform all year round, they will produce more in the summer months, when there is more light and the intensity is stronger, than the winter months.
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