HEM is a new methodology: The Home Energy Model is being developed from the ground up. This is to address the limitations of the decades-old SAP methodology, which was originally designed for manual, pen-and-paper use. HEM is more accurate: HEM offers a more accurate view of a home's energy consumption by simulating performance in 30-minute intervals, unlike SAP's monthly models. This detailed data delivers a highly precise understanding of energy demand and usage. HEM supports net-zero goals: The updated model now more accurately reflects modern technologies such as heat pumps, solar panels, and battery storage. It also incorporates forward-looking emission factors, ensuring that assessments align with the UK's increasingly decarbonized electricity grid. HEM is transparent: The code for the calculation engine is open-source, allowing for greater scrutiny, collaboration, and continuous improvement from the industry

HEM is composed of two main parts: Core calculation engine: The software package designed to model a building's physics, encompassing its heating systems, structural elements, and electricity generation Wrappers: Wrappers are used to define the purpose of the Home Energy Model (HEM) within a specific context. They adapt user inputs, input assumptions, and output metrics, allowing the HEM to be utilized for particular tasks. Each wrapper customizes the HEM core to suit its intended application.

Wrappers define the context and assumptions for a specific type of energy assessment, ensuring consistency and clarity. Future Homes Standard (FHS) assessment: The first wrapper, designed to ensure new homes comply with upcoming FHS regulations Energy Performance Certificates (EPCs): A separate wrapper is being developed for EPCs, which will be used for both new and existing homes to give more accurate ratings. Other potential applications: The modular design allows for future wrappers to be developed for other purposes, such as overheating assessments or performing whole-life carbon assessments.

More detailed input: Assessors and designers will need to provide more specific information, such as plot specific distant shading obstacles, ventilation areas, openable parts of windows and PV inverter details to get compliant results. Better-informed decisions: With more accurate, granular data, HEM will enable better reporting on energy efficiency and carbon emissions, helping homeowners, buyers, and renters make informed decisions. Phased transition: HEM is not an immediate change. For a limited time, both HEM and an updated version of SAP (SAP 10.3) will be used to allow for a smooth transition and further testing. New energy metrics: The model may also shift the metrics used on EPCs, on whose future a consultation has been undertaken and is likely to result in significant reform Higher complexity: The increased level of detail and half-hourly simulations make HEM assessments more complex to perform than SAP assessments. Longer runtime: The computational load is significantly higher, meaning simulations take longer to complete

Increased Time Resolution: SAP 10.2 uses monthly calculations, which limits its ability to model dynamic energy use. The HEM simulates energy performance every half-hour, allowing for better representation of heat pumps, smart technologies, energy storage, and load-shifting behaviors. International Standards: Unlike SAP, which was based on the older BREDEM methodology, the HEM is built upon BS EN ISO 52016-1:2017, aligning it with current international standards for calculating building energy performance. "Wrappers" for Different Use Cases: The HEM uses "wrappers" to define standardized inputs and outputs for specific functions, such as demonstrating compliance with the Future Homes Standard (FHS) or producing Energy Performance Certificates (EPCs). This separates the core building physics model from policy-specific assumptions, allowing for user-centered design, increased flexibility, and clear accountability. Revised Product Characteristics Database (PCDB): The existing PCDB will be rebuilt to be compatible with the HEM, incorporating additional data for improved accuracy of product characterization (e.g., boiler modulation ratios, backup heater power for heat pumps). Recognizing New Technologies (Replacing Appendix Q): The current "Appendix Q" process for incorporating new technologies is being reformed. The new process aims for increased transparency, robust evidence requirements, continuous evaluation, and greater integration with other innovation recognition processes. Existing Appendix Q technologies will be fully integrated into the HEM and PCDB. Modular Architecture: The HEM features a modular design, where different components (e.g., heating systems) are clearly delineated. This simplifies the model, makes it more flexible for continuous improvement, and allows for easier integration of new features. Energy Flexibility and Smart Technologies: The half-hourly simulation capability enables realistic modeling of interactions between variable energy supply and demand, quantifying benefits from storage and load-shifting technologies and behaviors. Open-Source Methodology: The HEM codebase has been published under the MIT Licence, aiming for open development and increased transparency. The government is considering using this open-source code as the approved methodology for regulatory uses. Changes to Software Delivery: The government is exploring a centralized, cloud-based version of the HEM calculation engine, which software providers can build user interfaces around. This aims to reduce inconsistencies, speed up changes, and stimulate innovation.