Home Energy Model Glossary — Key Terms Explained

Plain English

How well a building prevents uncontrolled air leaking in and out through gaps in the walls, floors, and roof. A more airtight home loses less heat.

Technical

The resistance of the building envelope to air infiltration, measured by a pressurisation test at 50 Pa and expressed as m³/(h·m²) of envelope area. The FHS is expected to require ≤ 8 m³/(h·m²) @ 50 Pa, with best practice targets of ≤ 3–5.

Plain English

A process in SAP that allows new or innovative technologies (not yet in the standard product database) to be included in energy calculations by providing special test data.

Technical

A mechanism within the SAP methodology for incorporating energy-saving technologies not covered by the standard calculation. Products require approved test data submitted through the Appendix Q process. This is being reformed as part of the transition to HEM.

Plain English

Air Source Heat Pump — a device that extracts heat from the outside air and uses it to heat your home and hot water. Works like a refrigerator in reverse.

Technical

An electrically driven vapour-compression heat pump that uses ambient air as its heat source. Performance (COP) varies with outdoor air temperature and the required flow temperature to emitters. In HEM, ASHP performance is modelled dynamically at each half-hourly timestep using EN 14825 test data.

Plain English

Building Research Establishment — the UK organisation that leads the development of HEM. BRE also originally created SAP in 1993.

Technical

The Building Research Establishment, a former government agency now operating as a private research and consultancy body. BRE leads the consortium developing HEM, commissioned by DESNZ. BRE maintains the Python reference implementation of HEM on Azure DevOps.

Plain English

The legal rules that set minimum standards for the design and construction of buildings in England and Wales. They cover things like structural safety, fire, and energy efficiency.

Technical

Statutory instruments made under the Building Act 1984 that set functional requirements for building work in England and Wales. Part L (Conservation of Fuel and Power) governs energy performance and is the primary regulation that HEM and the Future Homes Standard apply to.

Plain English

A number that represents how much CO₂ is produced per unit of energy from a given fuel. Electricity’s carbon factor has fallen dramatically as the UK grid has added more wind and solar.

Technical

The mass of CO₂ equivalent emitted per kWh of delivered energy, expressed in kgCO₂e/kWh. SAP (RdSAP 10) uses historical factors based on the 2012 grid mix. HEM uses forward-looking factors based on the projected 2025–2029 average, better reflecting grid decarbonisation.

Plain English

Coefficient of Performance — a measure of how efficient a heat pump is. A COP of 3 means the heat pump produces 3 units of heat for every 1 unit of electricity it uses.

Technical

The ratio of useful heating (or cooling) output to electrical energy input at a given operating point. COP varies with the temperature difference between source and sink. In HEM, COP is calculated dynamically at each timestep based on EN 14825 performance data, rather than using SAP’s fixed seasonal values.

Plain English

Department for Energy Security and Net Zero — the UK government department responsible for energy policy, including the Home Energy Model.

Technical

The government department that commissioned the development of HEM and is responsible for the SAP/HEM methodology, energy policy, and EPC reform. DESNZ published the HEM consultation and its October 2025 government response.

Plain English

Dwelling Fabric Energy Efficiency — a measure of how much energy the building’s walls, roof, floor, and windows allow to escape, independent of what heating system is used.

Technical

A compliance metric used in the FHS wrapper that quantifies the energy demand for heating and cooling attributable to the building fabric alone, measured in kWh/m²/year. It uses a standardised heating system so that the metric reflects only the quality of the thermal envelope.

Plain English

Domestic Hot Water — the hot water you use for baths, showers, washing up, and other daily tasks (as opposed to the water circulating in your heating system).

Technical

Water heated for sanitary purposes (bathing, washing, cooking). In HEM (HEM-TP-09), DHW demand is modelled using individual tapping events with specified flow rates and temperatures, rather than SAP’s monthly total derived from floor area.

Plain English

Dwelling Primary Energy Rate — a measure of the total primary energy a home uses, including the energy lost in generating and delivering that energy to the home.

Technical

A compliance metric in the FHS wrapper expressing the annual primary energy consumption of the dwelling in kWh/m²/year. The assessed dwelling’s DPER must not exceed the DPER of a notional building of the same geometry to demonstrate compliance.

Plain English

The temporary period during which both SAP 10.3 and HEM can be used to demonstrate compliance with the Future Homes Standard. This gives the industry time to transition.

Technical

The phased transition period confirmed by the government in May 2025, during which both SAP 10.3 and HEM are accepted compliance methodologies for the FHS. The duration of dual running has not yet been formally defined.

Plain English

Energy Calculation as a Service — the government’s cloud-based platform that runs the official HEM calculation. All assessors will use this single system instead of multiple competing software packages.

Technical

A cloud-based API operated by MHCLG that provides the authoritative implementation of HEM using the Rust performance engine. ECaaS will be the only valid means to confirm Part L compliance under the FHS. Software providers build user interfaces on top of the API. Contact: ECaaS@communities.gov.uk.

Plain English

Energy Performance Certificate — a rating (currently A to G) that shows how energy-efficient a property is. Required when selling or letting a home. HEM will eventually be used to produce more detailed EPCs.

Technical

A certificate rating the energy efficiency of a building, required under the Energy Performance of Buildings Regulations 2012 for sales and lettings. Currently produced using SAP (new builds) or RdSAP (existing homes). HEM-based EPCs are targeted for launch from October 2026, with four new headline metrics replacing the single A–G scale.

Plain English

A measure of how good a building’s physical structure (walls, roof, floor, windows) is at retaining heat — regardless of what heating system is installed.

Technical

The energy demand for space heating and cooling per unit floor area, attributable solely to the thermal performance of the building fabric. Assessed using a notional standardised heating system to isolate fabric performance from system efficiency. Proposed as one of the four new EPC headline metrics.

Plain English

A design approach that prioritises making the building itself as energy-efficient as possible (good insulation, airtightness, thermal mass) before adding technology like heat pumps or solar panels.

Technical

A design philosophy that prioritises reducing energy demand through the building envelope (U-values, airtightness, thermal bridging, thermal mass, solar gains management) before optimising energy supply systems. HEM’s dynamic modelling makes fabric first strategies more impactful because their timestep-by-timestep benefits are properly captured.

Plain English

Future Homes Standard — the upcoming update to building regulations that will require new homes in England to produce 75–80% less carbon than under the 2013 rules. It effectively requires heat pumps and solar PV.

Technical

The update to Part L of the Building Regulations for England, requiring 75–80% carbon reduction compared to 2013 standards. Key requirements include low-carbon heating (heat pumps), solar PV, enhanced fabric performance, and airtightness targets. HEM is the primary compliance assessment methodology, with SAP 10.3 available during a dual running transition.

Plain English

The ratio of a building’s external surface area to its floor area. A compact building (like a flat) has a low form factor and loses less heat. A detached house with many corners has a higher form factor.

Technical

The ratio of total thermal envelope area to total internal floor area. Lower form factors indicate more compact designs with less heat loss per unit of floor area. HEM’s half-hourly modelling makes form factor more influential because fabric heat losses are calculated dynamically. FHS draft guidance suggests targeting Form Factor < 3.

Plain English

Ground Source Heat Pump — a heat pump that extracts heat from the ground (via buried pipes) rather than from the air. Generally more efficient than air source but more expensive to install.

Technical

An electrically driven heat pump using the ground or groundwater as its heat source, typically via horizontal ground loops or vertical boreholes. Provides more stable source temperatures than ASHPs, resulting in higher average COP. In HEM, GSHP performance is modelled with ground temperature profiles varying by month and depth.

Plain English

A device that moves heat from one place to another using electricity. It can extract heat from the air, ground, or water outside your home and deliver it inside. Much more efficient than directly converting electricity to heat.

Technical

A vapour-compression or absorption cycle device that transfers thermal energy from a lower-temperature source to a higher-temperature sink. HEM models heat pump performance dynamically via HEM-TP-12, calculating COP at each half-hourly timestep based on source temperature, sink (flow) temperature, and EN 14825 / EN 15316-4-2 performance curves.

Plain English

Home Energy Model — the UK Government’s new method for calculating how much energy a home uses. It replaces SAP and uses half-hourly calculations instead of monthly averages.

Technical

The Home Energy Model, a dynamic thermal simulation methodology that calculates building energy performance at half-hourly timesteps across a full year (17,520 periods). Based on BS EN ISO 52016-1:2017. Developed by a BRE-led consortium, commissioned by DESNZ. Delivered through ECaaS. Open source under MIT Licence.

Plain English

Heat Transfer Coefficient — a single number that describes the total rate at which heat escapes from a building, combining losses through the fabric and from air leakage.

Technical

The overall heat transfer coefficient of a dwelling in W/K, representing the sum of fabric transmission losses and ventilation losses. Used as a key metric for assessing the thermal performance of the building envelope. SMETER technologies aim to measure HTC in-situ, though HEM does not currently support an external HTC replacement option.

Plain English

The uncontrolled leakage of air into and out of a building through gaps, cracks, and openings in the structure. Different from deliberate ventilation.

Technical

Uncontrolled air exchange through the building envelope driven by wind pressure and stack effect. In HEM (HEM-TP-06), infiltration is modelled using a pressure-driven approach based on BS EN 16798-7:2017, distinct from purpose-provided ventilation. Infiltration rates are influenced by airtightness test results and building geometry.

Plain English

Ministry of Housing, Communities and Local Government — the UK government department responsible for building regulations and the ECaaS platform.

Technical

The government department responsible for building regulations in England, including Part L. MHCLG operates the ECaaS platform and maintains the Rust implementation of HEM on GitHub. Also responsible for the Building Safety Regulator (BSR) under HSE.

Plain English

Mechanical Ventilation with Heat Recovery — a system that brings fresh air into a building while recovering heat from the stale air being extracted. Essential in very airtight homes.

Technical

A balanced mechanical ventilation system with a heat exchanger that recovers thermal energy from exhaust air and transfers it to incoming supply air, typically achieving 85–95% heat recovery efficiency. Required in practice for homes with airtightness below approximately 3 m³/(h·m²) @ 50 Pa to maintain indoor air quality under Part F.

Plain English

The point at which the amount of greenhouse gases produced is balanced by the amount removed from the atmosphere. The UK has a legal target to reach net zero by 2050.

Technical

A state in which anthropogenic greenhouse gas emissions are balanced by removals over a specified period. The UK’s legally binding target under the Climate Change Act 2008 (as amended in 2019) is net zero by 2050. The FHS and HEM are key instruments for decarbonising new housing in support of this target.

Plain English

A theoretical version of your building used as a benchmark for compliance. It has the same shape and size as the real building but uses standard reference specifications. Your building must perform at least as well.

Technical

A hypothetical building of identical geometry to the assessed dwelling but with prescribed reference fabric specifications, heating systems, and occupancy patterns. Under the FHS, the assessed dwelling’s DPER and DFEE must not exceed those of its notional building equivalent. The notional building specifications define the de facto performance standards.

Plain English

The section of the Building Regulations covering ventilation — ensuring homes have adequate fresh air supply for healthy indoor air quality.

Technical

Approved Document F of the Building Regulations, setting requirements for the means of ventilation in buildings. Updated alongside the FHS to address indoor air quality in highly airtight buildings. Requirements include minimum extract rates, purge ventilation, and background ventilators or mechanical systems.

Plain English

The section of the Building Regulations covering energy efficiency — the rules that determine how energy-efficient a new or renovated building must be. The Future Homes Standard is the next major update to Part L.

Technical

Approved Document L of the Building Regulations (Conservation of Fuel and Power), setting requirements for the energy performance of new and existing buildings. Part L1A covers new dwellings. The FHS represents the next major revision, requiring 75–80% carbon reduction compared to 2013 Part L. HEM and SAP 10.3 are the approved compliance methodologies.

Plain English

The section of the Building Regulations covering overheating — ensuring that highly insulated and airtight homes don’t become uncomfortably hot in summer.

Technical

Approved Document O, introduced in June 2022, setting requirements to limit overheating risk in new residential buildings. Addresses the risk that highly insulated, airtight buildings may overheat, particularly with large south-facing glazing areas. HEM’s dynamic half-hourly simulation provides a full overheating assessment, replacing SAP’s simplified check.

Plain English

Product Characteristics Database — a database of heating, hot water, and ventilation products with their official performance data, used in energy calculations.

Technical

The Product Characteristics Database maintained for SAP/HEM calculations, storing manufacturer-specific performance data for heating appliances, hot water systems, ventilation equipment, and other energy-related products. Being revised for HEM. The HEM consultation response (87 respondents, 54 supporting revision) indicated strong support for a streamlined product recognition process.

Plain English

The total energy consumed to deliver energy to your home, including the energy lost in generating electricity at power stations and transporting gas through pipes. It’s always more than the energy you actually use.

Technical

The total energy input required to deliver useful energy to the building, accounting for upstream conversion and distribution losses. Expressed using primary energy factors (PEFs) applied to each fuel type. HEM uses updated PEFs reflecting the current and projected generation mix, in contrast to SAP’s older factors.

Plain English

Photovoltaics — solar panels that convert sunlight into electricity. Most new homes under the Future Homes Standard will be required to have them.

Technical

Photovoltaic panels that convert solar irradiance into electrical energy. In HEM (HEM-TP-18), PV generation is calculated at each half-hourly timestep using actual solar irradiance data, panel orientation, tilt, and inverter specifications. Self-consumption — the proportion used on-site versus exported — is modelled, as is battery storage charge/discharge behaviour.

Plain English

Reduced Data SAP — a simplified version of SAP used for producing EPCs for existing homes, where full construction details aren’t available. An assessor visits and measures what they can, and defaults fill the gaps.

Technical

A reduced data version of the SAP methodology designed for assessing existing dwellings where full construction data is unavailable. Uses surveyor observations and age-based defaults. Currently the most common method for domestic EPCs in the UK. Eventually to be replaced by a HEM-based reduced data methodology for existing dwellings.

Plain English

Standard Assessment Procedure — the UK’s current method for measuring how much energy a home uses, first published in 1993. Being replaced by the Home Energy Model.

Technical

The Standard Assessment Procedure for Energy Rating of Dwellings, the UK Government’s methodology for assessing the energy performance of domestic buildings. Uses monthly steady-state calculations. Current version is SAP 10.2. Being replaced by HEM for regulatory compliance.

Plain English

The current version of SAP, with updated carbon factors and some technology improvements. Still uses the monthly calculation approach.

Technical

The current approved version of SAP, updating fuel prices, carbon emission factors, and some technology modelling compared to SAP 2012. Maintains the monthly steady-state calculation approach. Used for current Part L 2021 compliance.

Plain English

An updated version of SAP that works with the Future Homes Standard. Available as an alternative to HEM during the transition period.

Technical

An interim update to SAP incorporating FHS notional dwelling specifications and revised primary energy and carbon factors. BRE approved the SAP 10.3 engine in late 2025. Available for FHS compliance during the dual methodology transition period alongside HEM.

Plain English

The proportion of electricity generated by your solar panels that you use directly in your home, rather than exporting it to the grid. Using your own solar electricity saves more money than exporting it.

Technical

The fraction of on-site PV generation consumed within the dwelling at the point of generation. HEM calculates self-consumption at each half-hourly timestep by comparing PV output with concurrent electrical demand. Battery storage increases self-consumption by storing surplus generation for later use. SAP cannot model self-consumption because it lacks sub-monthly time resolution.

Plain English

The heat that enters a building through windows and the building fabric from sunlight. Helpful in winter (free heating) but can cause overheating in summer.

Technical

Thermal energy transmitted through glazing and absorbed by opaque building elements from solar irradiance. In HEM (HEM-TP-08), solar gains are calculated hourly using direct and diffuse irradiance components, considering window orientation, tilt, shading, and fabric solar absorption — a significant improvement over SAP’s monthly, window-only approach.

Plain English

A part of the building where heat escapes more easily than through the surrounding structure — for example, where a wall meets a floor, or around a window frame. Good design minimises these weak points.

Technical

A localised area of the building envelope with significantly higher heat transfer than the surrounding elements, occurring at junctions between building components (linear thermal bridges, measured as psi-values in W/mK) or at discrete points (point thermal bridges, measured as chi-values in W/K). In HEM, thermal bridge losses are calculated at each timestep rather than as an annual uplift.

Plain English

The ability of a building’s materials to absorb and store heat. Heavy materials like concrete and brick have high thermal mass — they warm up slowly but also release heat slowly, helping to keep temperatures stable.

Technical

The capacity of building materials to store thermal energy, characterised by the product of density, specific heat capacity, and thickness. In HEM (HEM-TP-07), thermal mass is modelled dynamically at each timestep using the thermal capacitance of each building element, enabling accurate prediction of temperature swings and heating demand profiles. SAP uses a simplified thermal mass parameter.

Plain English

A measure of how quickly heat passes through a building element like a wall, roof, floor, or window. Lower U-values mean better insulation. Measured in W/m²K.

Technical

The thermal transmittance of a building element, expressed in W/m²K, representing the rate of heat transfer per unit area per degree of temperature difference between internal and external environments. Calculated in accordance with BS EN ISO 6946:2017. FHS expected U-values: walls ≤ 0.15–0.18, floors ≤ 0.11–0.13, roofs ≤ 0.11–0.13, windows ≤ 0.8–1.2 (triple glazing).

Plain English

The process of bringing fresh air into a building and removing stale air. Can happen naturally (through windows and vents) or mechanically (using fans or MVHR systems).

Technical

The supply and extraction of air to and from a building for indoor air quality purposes. In HEM (HEM-TP-06), ventilation is modelled using a pressure-driven approach based on BS EN 16798-7:2017, considering purpose-provided openings, extract fans, MVHR systems, and infiltration paths separately. Wind pressure and stack effect drive airflow at each timestep.

Plain English

A set of policy rules that sits on top of HEM’s core physics engine. Different wrappers are used for different purposes — the FHS wrapper checks compliance with building regulations, the EPC wrapper will produce Energy Performance Certificates.

Technical

A policy-specific configuration layer applied to HEM’s core calculation engine, defining standardised assumptions, compliance metrics, and post-processing rules for a particular regulatory purpose. The modular architecture allows the core engine to be updated independently of individual wrappers, and wrappers to evolve independently of each other.

Plain English

A home designed so that it will automatically reach net zero carbon emissions as the electricity grid gets cleaner — without needing further retrofitting. This is the goal of the Future Homes Standard.

Technical

A building designed with fabric performance, low-carbon heating, and on-site generation such that it will achieve net zero operational carbon emissions as the electricity grid fully decarbonises, without requiring further physical intervention. The FHS’s 75–80% carbon reduction target using forward-looking emission factors is designed to deliver zero carbon ready homes.