HEM Technical Reference — Deep Dive into the Home Energy Model

The Home Energy Model (HEM) is a half-hourly dynamic simulation engine that calculates the energy performance of domestic buildings. Built on BS EN ISO 52016-1:2017, it models heat balance, ventilation, solar gains, and system performance at 30-minute intervals — replacing SAP's monthly steady-state approach with a physics-based simulation capable of accurately representing modern low-carbon technologies.

HEM Architecture

HEM uses a modular architecture that separates the core building physics engine from policy-specific wrappers. The core engine performs the fundamental energy simulation, while wrappers apply the rules, assumptions, and compliance metrics required for specific regulatory purposes.

Core Engine

The core engine is a deterministic simulation that takes a building description (geometry, fabric, systems) and external conditions (weather data) as inputs, then calculates energy flows at half-hourly intervals for a full year — producing 17,520 timesteps. The engine is modular, with separate calculation modules for each physical process:

• Fabric heat loss (HEM-TP-05) — walls, floors, roofs, windows, doors
• Thermal bridges — linear and point thermal bridges at every timestep
• Ventilation & infiltration (HEM-TP-06) — pressure-driven model based on EN 16798-7
• Thermal mass (HEM-TP-07) — dynamic thermal storage in building fabric
• Solar gains (HEM-TP-08) — direct and diffuse solar radiation through glazing and fabric
• External conditions (HEM-TP-03) — weather data and hourly sun position calculation
• Space heating & cooling demand (HEM-TP-04) — heat balance equations per zone
• Domestic hot water (HEM-TP-09) — individual tapping events and stratified storage
• Heat pumps (HEM-TP-12) — variable COP with source/sink temperatures
• PV & battery storage (HEM-TP-18) — generation, self-consumption, and battery behaviour
• Heat emitters (HEM-TP-16) — radiators and underfloor heating performance
• Controls (HEM-TP-17) — heating controls and smart thermostats

Policy Wrappers

Policy wrappers sit on top of the core engine and apply context-specific rules:

• FHS wrapper — applies the Future Homes Standard notional building specification, compliance metrics, and carbon emission factors for new-build compliance
• EPC wrapper (under development) — applies assumptions for Energy Performance Certificates, including reduced data methodology for existing homes
• Future wrappers — the modular architecture allows additional wrappers for other policy uses without changing the core physics

Core Calculation Loop

For each of the 17,520 half-hourly timesteps in a year, HEM executes the following sequence:

1. Hot water demand — calculate domestic hot water demand from tapping events and determine energy supplied by the heating system (including cylinder losses and pipework losses)
2. Space heating/cooling demand — calculate the heat balance for each zone considering fabric losses, ventilation losses, solar gains, internal gains, and thermal mass effects
3. System interactions — additional calculations for multi-service systems (e.g. combi boilers serving both space heating and hot water)
4. Electricity balance — calculate PV generation, self-consumption, battery charge/discharge, and grid import/export
5. Results aggregation — return timestep results to the active wrapper for post-processing against policy targets

This loop structure means that every interaction between building fabric, weather, occupancy patterns, and mechanical systems is captured at a resolution that reveals real-world performance characteristics invisible to monthly calculations. For the full methodology, see How HEM Calculates.

SAP vs HEM — Technical Comparison

The shift from SAP to HEM represents a fundamental change in calculation methodology. The table below summarises the key technical differences:

For a non-technical comparison, see SAP vs HEM. For a comparison focused on FHS compliance routes, see SAP 10.3 vs HEM.

HEM Technical Papers

The HEM calculation methodology is documented in a series of technical papers published alongside the HEM technical documentation on GOV.UK. Each paper covers a specific calculation module:

Implementation & Delivery

HEM is delivered through two complementary channels:

ECaaS Platform

The ECaaS (Energy Calculation as a Service) platform is a cloud-based API run by MHCLG that provides the official HEM calculation for statutory purposes. ECaaS will be the only valid means to confirm compliance with Part L when using the HEM route — replacing the current model where multiple software providers build their own SAP engines.

Open Source Codebase

The HEM source code is published under the MIT Licence (Crown Copyright) in two implementations:

• Python (BRE, Azure DevOps) — the reference methodology used for ongoing development and validation
• Rust (MHCLG, GitHub) — the performance-optimised implementation that powers ECaaS

For full details on the codebase structure and how to contribute, see Open Source Guide.

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