The solar PV blog

The Standard Assessment Procedure (SAP) is used to assess the energy performance of dwellings and includes a section to estimate the annual performance of a solar PV system. PV installers should be very familiar with the PV – SAP calculation as it is the method currently proscribed under the Microgeneration Certification Scheme (MCS).

A proposed revision to SAP – SAP 2012 – is out for comment at the moment >> SAP 2012

The most significant change to the solar PV performance calculation procedure in the draft of SAP 2012 is a wholly new means to obtain the annual solar radiation for the site in question. Previously, SAP made no allowance for geographic location and a simple look-up table gave the values as shown below:

 SAP 2012 provides 21 different zones around the UK and proscribes a series of steps to generate the solar radiation value for each of these areas. As such it provides a much more region specific value for solar radiation. Examples for a couple of these zones in the same tabular format as the present SAP-2009 table are shown below (calculated from the draft SAP2012 procedure):

While this change should provide more accurate results, it is in my opinion spoilt by the retention of my least favourite table – table H2 – the Overshading factor. Solar PV systems are significantly affected by shade, so shade considerations form a big part in the assessment of any PV site. The SAP table simply allows for the selection of 4 options:

• None or very little shade (<20% of sky blocked by obstacles)
• Modest shade (20-60% of sky blocked by obstacles)
• Significant shade (>60% -80% of sky blocked by obstacles)
• Heavy shade (<80% of sky blocked by obstacles)

The “percentage of sky blocked by obstacles” is a very poorly defined term and in our experience is both very hard to determine and also very open to miss-use or variations in interpretation. Retaining this simplistic assessment of shade for PV systems seems at odds with the now improved methodology for assessing the solar radiation.

Solutions are of course difficult. We are currently trying to define a procedure for the new PV guide – see my earlier post.

The SAP 2012 consultation closes on 28 March.