Wednesday, 12 June 2013

SAP 2012 Gives Solar Heating a Boost

What the Government's new Calculation Means for Solar

Credit: Viridian Solar

Ahead of the announcement of the new Building Regulations later this summer, BRE has published the accompanying energy calculation, the Standard Assessment Procedure, or SAP.  

If you think this document is only relevant to the new build sector, you would be wrong. SAP is also the basis for Energy Performance Certificates (EPCs), and therefore is also highly influential in the refurbishment of existing homes, for example under the Green Deal. SAP Appendix H is also used by the Microgeneration Certification Scheme (MCS) for installers of solar heating to provide customers with an estimate of their likely energy savings. Finally, whichever of the proposed routes to calculate payments under the forthcoming incentive scheme for renewable heat, the domestic RHI, is chosen, SAP lies behind it somewhere.

This document is not for the faint-hearted, comprising a thick booklet of dense calculations and notes. Fortunately the solarblogger has done the heavy lifting, so you don't have to...

A detailed assessment has been published as a briefing document available from the Viridian Solar website, but here's a quick round up of the headlines:

 New postcode-based irradiation data (PV&T)
The UK average irradiation remains the same, but the value varies depending on location with northern Scotland around 10% below the average and southern England around 10% above.  This feeds directly into the solar pv estimate, but results in a smaller +/- 5% change across the country for solar heating.

► Updated fuel carbon emissions factors (PV&T)
Carbon savings from solar electricity reduce by 2%, carbon emissions avoided by solar thermal replacing natural gas increase by 9%

 Addition of hot water use factor (T)
There is a 29% increase in hot water used from the hot water cylinder in the solar thermal calculation if no electric showers are present, and a reduction of 36% if only electric showers are present. This produces a 20% increase in solar heating energy in SAP Appendix H compared to the previous version

 Reduction in solar pump electricity consumption (T)
The 75kWh flat rate for the solar pump electricity consumption is reduced to 50kWh for mains powered solar pumps, based on evidence from EST solar thermal field trials submitted by EST and Solar Trade Association

 Addition of second order coefficient for thermal panel efficiency (T)
Creates a level playing field (for more information see my paper on this issue here)

So What Does it all Mean?


For solar PV, the main impact will be on new build housing.  Here the changes are relatively neutral, being a 2% drop in carbon savings on average due to the lower emissions factor for solar electricity. Of course, compared to the previous version of SAP, which used a single figure for irradiation for the entire country, some areas have increased significantly while others have fallen. (See map, below)

Credit: Viridian Solar

For retrofit PV the recent MCS PV installation Guide has already implemented a new energy calculation based on a different set of  irradiation data.  This produces slightly higher energy outputs than SAP 2012.  The discrepancy could be explained by the different uses of each calculation, with the MCS method taken to predict year one energy (with solar panel performance degradation taken into account later for any financial forecasts).  By contrast, SAP is more aimed at producing a whole life average estimate.

Solar Thermal

For solar heating, the calculation impacts both new build and retrofit, and in a really positive way. (See map, top of post)

In new build, carbon savings will increase significantly for almost all homes with solar heating.  The new adjustment factor for electric showers rightly produces a disincentive to install these alongside solar heating.  I reckon that overall the boost to solar heating carbon savings from all the changes is worth between 17% and 31% depending on where the house is.

In retrofit situations, the MCS installer will perform an energy calculation  for the customer using SAP Appendix H. The new version of SAP will an average of a 20% boost to the solar energy estimate, again differing depending on the location of the installation.  

Further changes are coming down to road to improve things even more in MCS, more on this soon....


  1. As the scope of SAP 2012 reminds us, SAP calculations are not intended for design purposes. Nor are the results of a particular Appendix such as H or M intended for any use other than input in thee main SAP calculation. The continued requirement for the installer to present such figures (often manually calculated) to end-users when they then must do separate design calculations to properly size systems remains a confusion to the public and a millstone round the neck of UK industry. Despite the welcome increase in sensitivity to the type of DHW appliances, I would stress the implications of using SAP for design particular for any non-domestic is a recipe for disappointment. Differing DHW regimes especially considering holiday periods are a key factor to be considered.

  2. I thought it was a while since you last posted, I guess you have an excuse if you had to digest SAP 2012...

    Great to see the calcs taking electric showers into account at last. I also have to agree with Chris that the need to get an EPC / GDA and/or SAP (+ PHPP?) and then separately get a new calculation done for your MCS accredited heat pump seems pointless.

    When you refer to solar heating I'm guessing you are only referring to DHW rather than space heating? Does the new SAP allow for taking solar thermal space heating into account at all? (Sorry, I know I could read it myself but like you say, you have done the heavy lifting already ...!)

    1. This comment has been removed by a blog administrator.

    2. Ben,

      No, there are no changes to include space heating contribution from solar thermal in the new SAP. I'm aware of one company considering a SAP Appendix Q approach for their combined water/space heating kit.

      What we really need is some convincing data to support a calculation for solar space heating. The Energy Saving Trust monitored trial was hoped to provide this - there were a few houses with such equipment in the study and an intention to produce a report on these separately. I suspect that budget constraints have done for that.

      Anyone know of any credible data out there that could underpin an approach to BRE to include this applicaton of solar heating?


  3. This system requires large storage battery to store adequate electricity to run your air-conditioners for specified period of time. Otherwise it requires a large area of solar panels to meet the demand. Lee