What is the Home Energy Model (HEM) and does it replace SAP?

The Home Energy Model (HEM) is the UK Government's next-generation methodology for assessing the energy performance of domestic buildings. It replaces SAP with a half-hourly dynamic simulation of energy use, carbon emissions, and running costs. SAP was first introduced in 1993 and last updated in 2012, making it an outdated tool — particularly as the methodology now needs to drive the property sector toward decarbonisation. 

HEM introduces several significant improvements over SAP. These include half-hour energy modelling intervals (compared to SAP's monthly averages), thermal zoning for detailed heat loss calculations per section of a building, and the integration of localised weather data. Solar gains, for example, are now calculated hourly based on the actual position of the sun combined with building orientation and shading objects — a far more accurate approach than SAP's simplified monthly figures. 

In terms of timing, when the FHS launches, both SAP 10.3 and HEM will be available as compliance routes as part of a phased transition, with SAP 10.3 serving as the interim methodology until HEM is fully deployed. HEM is expected to follow the FHS launch by a minimum of three months.