The SAP energy rating of a home is the most widely used indicator of domestic energy efficiency in the UK. It is used on domestic Energy Performance Certificates (EPCs) and (usually expressed as an average) as a performance indicator for housing stocks.
The Standard Assessment Procedure (SAP) defines a mathematical model that is used to predict the energy consumption in a dwelling, based on standard assumptions about occupancy patterns (hours of heating, room temperatures, etc.) and typical weather conditions. This estimated energy consumption can then be used to estimate annual energy costs and the associated carbon dioxide emissions according to the fuel(s) used.
The SAP energy rating is a score between 1 and 100 (or more) based on the dwelling’s estimated annual cost for heating, hot water and lighting. The higher the rating the better, with 100 defined as net zero energy costs. Although often referred to as an energy efficiency rating, it is more accurate to describe it as an energy cost rating.
The use of energy cost to define the SAP energy rating is usually justified on the basis that it is the cost that the occupant of the home is most likely to be concerned about. This is undoubtedly true but there are a number of consequences of this decision that arise from the fact that fuel tariffs change over time (usually increasing!). To avoid this leading to constantly changing energy ratings, the fuel tariffs used are fixed for periods of 3 or 4 years between updates of the SAP. However, the prices of different fuels tend to change at different rates, which means that when the energy rating scale is updated, properties heated by some fuels increase while properties heated by other fuels decrease. (When SAP was first introduced in 1995, it was expected that these relative fuel price changes would cancel each other out over a period of 3 – 4 years but this hasn’t proved to be the case.)
Since the introduction of EPCs, there have been two updates of SAP – SAP 2009 and SAP 2012. When SAP 2009 replaced SAP 2005 for EPCs (from April 2011 for existing dwellings), the energy ratings of gas heated properties decreased but those for properties heated by electricity, solid fuel (including biomass) or LPG increased quite significantly, with only oil heated properties remaining almost the same. However in the most recent update, when SAP 2012 replaced SAP 2009 (in December 2014 for existing dwellings), the energy ratings of properties heated by electrify, oil, LPG and solid mineral fuel decreased, although this time there was no significant change for properties heated by mains gas.
UK Domestic Energy Price Indices – Trends: January 2007 to September 2015; source: https://www.gov.uk/government/statistical-data-sets/monthly-domestic-energy-price-stastics
Since 2012, the price of oil in particular has fallen significantly, so the current energy rating of oil heated properties is lower than it should ideally be. (I recently paid 33p per litre whereas SAP 2102 assumes about 55p per litre). Note however that this relates only to the energy rating itself, not to the predicted energy costs and potential savings from improvements that are quoted on the EPC, which are based on average tariffs that are updated every 6 months.
While changes in fuel tariffs are generally the biggest factor in any changes to energy ratings, there are often also other changes introduced when SAP is updated that may affect the energy ratings for some property types. These are usually the result of attempts to make the procedure more accurate in the light of latest research or the introduction of new technologies. An example which may lead to a lower energy rating is the inclusion of heat loss via a cavity party wall in SAP 2012.