What are the key changes between the SAP 10.2 and HEM methodology?

The Government intends to replace the Standard Assessment Procedure (SAP) with the Home Energy Model (HEM) when the Future Homes Standard is introduced in 2025. In future, HEM could be the route by which new homes demonstrate compliance with Part L and Energy Performance Certificates (EPC) are generated for all homes.

There are key changes within the calculation methodology between HEM and SAP 10.2. Some of these areas include increased time resolutions, alignment with international standards, the architecture and the way energy flexibility and smart technologies are modelled.

Increased time resolutions

Energy demand and building services performance are calculated using timesteps – set data predicting use/performance at specific times. SAP 10.2 currently uses monthly timesteps, meaning energy demand and building services performance are evaluated in twelve snapshots across the year.

Having such low frequency timesteps has drawbacks, especially for heat pumps where performance can vary overtime depending on various factors. As SAP 10.2 is an older, simpler tool, heat pumps have to currently be simulated outside the main SAP model so they can use a higher time resolution, which prevents them from interacting properly with other technologies or systems present in the model.

By comparison, the new Home Energy Model can simulate homes at a 30-minute time resolution. This means that every half-hour of the simulated year is evaluated, with specific exterior conditions and a memory of the situation in the previous timestep.

Increasing the time resolution of the model has several benefits, including:

  • Fewer constraints: Enabling half-hourly simulation gives the flexibility to choose the level of detail in which to model each element of the building.
  • Improved representation of heat pumps: Heat pumps are now simulated in a home’s specific context, rather than as a generic test simulation.
  • Smart technologies and storage: Increasing the time resolution means the Home Energy Model is better able to model the benefits of smart technologies, energy storage, and load-shifting. A reduced timestep also improves the treatment of energy generation (e.g. solar PV panels) and will enable the Home Energy Model to accommodate a wider range of energy tariffs. Peak demand can also be considered more accurately.
  • Supporting different use cases: Increasing the time resolution will help support all possible use-cases of the Home Energy Model.
  • Validation against dynamic simulation and monitoring data: Increasing the time resolution enables detailed, meaningful comparison between the Home Energy Model and more advanced dynamic simulation models of building physics, as well as between real time monitoring data from specific buildings. This helps to further improve confidence in the accuracy and flexibility of the model.

Alignment with international standards

During a recent SAP 10 review, alignment with current British and European Standards was chosen as the most effective method to quickly develop a new model which could achieve a high level of confidence and quality assurance, while retaining the required level of ambition.

In combination with the decision to increase the time-resolution, this has necessitated a move away from BREDEM, which is no longer the basis for the Home Energy Model. Government still reserves the option to deviate from the European Standards in places where they have evidence or a basis for doing so (for example, from lab testing data). Chapter 5 within the Homes Energy Model consultation shows the methodological approach for standards taken for individual technologies and systems within HEM

A modular architecture

As a long-standing system, SAP has been continuously developed and its functions have expanded over time, leading to a lack of structural clarity. Building the Home Energy Model from scratch has provided the opportunity for a clean slate to redesign the structure of SAP using wrappers and a modular architecture.

The Home Energy Model reference code is constructed of a modular framework. The boundary of a module is defined by a fixed set of inputs and outputs that are exchanged with other modules, which should not typically vary when the inner workings of the module change. Therefore, one e.g. heating system could be switched for another without the rest of the code having to change, because the two systems share a common format despite their internal calculations and parameters being different.

There are several given benefits to adopting this approach:

  • Ease of use: Having clearly delineated modules within the Home Energy Model simplifies the model and helps remove barriers to understanding. It also becomes easier to identify and isolate errors.
  • Flexibility and continuous improvement: New features can be added within modules or as new modules, without needing to update the rest of the code. It may be that different organisations could develop different modules in parallel in the future. The model’s architecture is expected to continue to expand and evolve throughout its development.

Modelling energy flexibility and smart technologies

Government notes energy flexibility as the ability to adapt and regulate energy use, generation, and/or storage in response to signals from the home or wider energy system.

As a monthly simulation, SAP 10.2 has limited ability to evaluate overlapping profiles for energy demand, weather, and fuel prices in order to assess the benefits of energy storage technologies or renewable electricity generation.

In comparison, HEM can run as a half-hourly simulation and can accept varying profiles, which potentially take unique values for every half-hour of the year. This permits realistic modelling of the interactions between variable supply and demand and enables the quantification of benefits from storage and load shifting technologies and behaviours. For example, the likely reduction in (daily or annual) peak electricity demand can be assessed, as can potential energy savings when switching to a time-of-use electricity tariff in combination with a solar PV diverter.

What next?

Government notes that HEM is still in development, and it will take time to review the changes and model scenarios to see whether the changes achieve their aim.

BEAMA hope to see that these changes are beneficial for smart technologies, heat pumps and solutions which enable flexibility.

If you have comments on the changes in HEM compared to SAP, please send your position to your group lead or Amy Collins using our member feedback form.

To read more on the Future Homes and Building Standards and the Home Energy Model, visit our central resource page.