Scottish Energy Performance Certificates - new metrics: research

Introduction

BRE were commissioned by the Scottish Government (SG) to undertake analysis work to help inform their decision making in relation to the potential use of a new Energy Performance Certificate (EPC) metric. This report gives a narrative of the work undertaken by BRE and some key results and recommendations from it. Further context is given in SG’s Heat in Building Strategy^[2].

All UK EPCs currently use as their headline rating a metric based on the predicted running costs of the dwelling, as modelled by the SAP or RdSAP methodology^[3], commonly referred to as the ‘SAP rating’ or ‘EPC rating’. This is expressed as a positive whole number, and an A to G rating. The metric behind the numerical rating is the modelled running costs per m² of floor area, which means large homes are not penalised relative to smaller ones. This is then mapped onto 1-100 scale^[4], then onto an A-G scale.

The running costs used are the modelled fuel costs for space heating/cooling, water heating, lighting, and pumps and fans associated with heating and ventilation systems. Energy use for domestic appliances is not included. Energy generated by on-site renewables (e.g. PV) is credited when the running costs are calculated, meaning it is possible to achieve low net running costs, and therefore a good SAP rating, even where the home is poorly insulated if enough renewables generation is added.

The use of a cost-based metric has worked reasonably well as a multipurpose metric in the past, when fuels with a high cost per kWh also tended to have a high carbon factor, meaning that using a rating which encouraged householders to take actions to reduce their running costs would also encourage carbon reductions. However, in recent years with the rapid decarbonisation of the electricity grid, this correlation has broken down to the extent that a cost-based metric now favours some relatively high carbon outcomes, particularly the continued use of fossil fuels over electric heating systems, contrary to the target to eliminate direct emissions heating by 2045 referred to in the Heat in Buildings Strategy. In the context of zero emissions heating systems the key focus then becomes demand minimisation and therefore a metric reflecting this is more appropriate.

SG is therefore considering creating a new EPC metric focussed only on the energy requirement of homes, for use alongside policy levers that ensure low carbon systems will be used to provide that energy.

Work undertaken by BRE in support of SG’s decision making on this subject has examined options for a suitable energy requirement metric and modelled the likely impact this would have on the distribution of ratings in the Scottish Housing stock, relative to the current rating system.