the solarblogger: The “Home Energy Model”



Alongside its consultation on the Future Homes Standard building regulations, the government
has revealed sweeping changes to the calculation underpinning Part L of the
building regulations (Conservation of Fuel and Power), and launched a consultation on the new approach.

The StandardAssessment Procedure (SAP) has been the government approved methodology to estimate
the energy performance of homes in the UK since 1993, a time when it was felt to be important that the method be simple enough to be completed with pen and paper and calculator. 

computer software emerged to make the job of energy assessors more convenient.  Provided by third party companies, these
applications needed to be checked by the Building Research Establishment (BRE),
the body responsible for the development of SAP, before they could be used to
demonstrate compliance with building regulations.

Over subsequent
versions of SAP issued in 1998, 2001, 2005, 2009, 2012 and 2022 the complexity of
the model increased.  Building elements were
dealt with in more sophisticated ways to improve the accuracy of the model (for
example the treatment of junctions in thermal insulation or – a personal
favourite – an improved treatment of the performance of solar thermal systems in
2005).  New technologies were more widely
adopted in construction and needed to be added (for example battery storage in 2022).

SAP’s Growing

of this approach had begun to emerge over time but were brought into sharp focus by
the 2022 implementation.  Developers found
themselves struggling to work out how to build new homes that complied with the
new building regulations already in force even as the third-party software was unavailable
due to delays in the certification process.

reason for a wholesale review of the model, flagged by the Climate Change Committee, was the emergence of key technologies that couldn’t be easily or accurately
added to the existing framework in a timely way:

  • Solar PV and
    self-consumption of generated electricity
  • Battery
    storage of electricity
  • Solar PV
  • Time of use
    energy tariffs
  • Smart energy
    controls – timing the use of energy to coincide with cheap tariffs and the
    availability of renewable energy.

model was based on a monthly time resolution. 
This meant that the impact of new technologies had to be demonstrated in
real life studies and an average performance across multiple households derived
before a simplified month by month impact could be added to the model.

A case in point
was the introduction of battery storage in the 2022 version.  Data was scarce because the technology was
relatively new.  A simplified average performance
that linked installed solar capacity, total energy use and battery storage
capacity was derived by applying a line of best fit to the available data.  This formula was incorporated into the
monthly SAP model.  No data was available
on homes that combined battery storage with solar PV diverters, so SAP only
allowed one of the technologies to be used at a time.


 A New Approach

The government
is consulting on a fundamental re-working of the model.  It’s such a big change that SAP has been
dropped in favour of a new name ‘The Home Energy Model’.  Changes include:

  • The model will
    be available as a cloud-based software ‘core engine’, with the source code
    published on GitHub
  • ‘Wrappers’
    will be published for different applications – comprising different starting
    assumptions as inputs which will then feed into the same core engine.  The first wrapper to be published will be for
    the 2025 building regulations, followed by a wrapper for the generation of
    Energy Performance Certificates (EPCs) for existing homes.
  • The
    software runs on a 30-minute time resolution, allowing better modelling of
    smart technologies such as solar, battery storage and time-of-use energy
  • An updated solar
    PV generation calculation is based on the hourly methodology in BS EN
    15316-4-3:2017, which includes the effect of ventilation on the rear of the panels.

on Solar


The solar
industry should welcome the change from SAP to the new Home Energy Model.

The move to
cloud-based software brings the approach up to date.  The separation of a core engine, based on
best available building physics modelling, from the ‘wrappers’ which clearly
surface the assumptions and inputs into the model for specific applications
such as building regulations or EPCs makes it much easier to interrogate how
the ‘black box’ is working.

The change
to a half-hour resolution better supports enabling technologies that work with solar PV – battery storage, smart energy controls and hot water from PV fed
immersion heaters.  This will further cement
the position of solar PV as a normal part of any new home built in the UK.

The change
to the hourly methodology for solar generation needs to be carefully assessed,
I will be writing about this in my next blog. 









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