Jersey Energy Performance Assessments (FOI)

​​​Request

Please provide the details of or a copy of the methodology that underpins Jersey Energy Performance Assessments, both qualitative and quantitative,  that sets out how a property achieves a certain rating.

Response

Energy Performance Certificates (EPC) tell you how energy efficient a property is and gives it a rating in respect of its energy cost and its carbon emissions ratings.  The assessment is made using, in the case of domestic properties, a Reduced Data Standard Assessment Procedure (rdSAP) and, for non-domestic properties, a Simplified Building Energy Model (SBEM).  At present, the use of SBEMs for non-domestic properties is very rare, therefore the focus of this response is based on the domestic property approach using the Jersey rdSAP.

All assessments of domestic properties are undertaken using the Jersey rdSAP modelling tool.  This has been developed in conjunction with the UK Building Research Establishment (BRE) who developed the various modelling tools used within the UK.  The Jersey version is nuanced to reflect specific differences applicable to Jersey, such as weather data, energy costs and building control standards.

The rdSAP modelling tool is a computer based interactive tool which allows accredited assessors to model a building’s energy efficiency.  This is done by inputting a range of values specific to each building being assessed.  These include dimensions, construction materials and types, products used within this building, including key elements such as heating types and specific product models.  Because it is a “reduced data” version of the SAP, there is scope for the assessment to use the tool’s in-built assumptions where evidence of the building’s attributes is unknown.  

Hence, once all the information which the assessor can reasonably determine has been gleaned and input into the rdSAP system tool, any gaps are assessed using default values from within the system’s baseline dataset.  These default values will be based on key building information such as the building’s age and construction type and will be assumed to reflect the standards of construction applicable to the age of the building.

Assessors can input data that they can properly assess without the need for any intrusive investigations.  The more data that they can gather about the construction of the building, the less reliance there will be on the default values provided within the tool.

The tool produces both an energy cost rating and a CO2 emissions rating, both of which are banded as attached below:

Energy ratings.pdf​​​

The ratings are based on a set of standard assumptions which may differ from how the building is actually being used.  For example, the number of people in the household is determined by the number of bedrooms in the dwelling.  This will give an indicative assessment of the number of people likely to reside in the building, as opposed to that actual number of residents at any one time.  This will impact on aspects of the assessment such as heating and hot water demand, electricity use and thermal gain.  This may differ from the actual occupation of the building, but as the assessment is about the building’s ability to use and retain energy, baseline assumptions are made to allow for a consistent approach.

The rdSAP tool is a complex engine with computer based coding used to calculate the outputs based on a wide range of technical inputs and inbuilt assumptions.  It is not possible to reproduce this in this format.

The following methodology documents do, however, provide furthermore detailed information about the data and assumptions used in the calculation of the ratings.  

The Government’s Standard Assessment Procedure for Energy Rating of Dwellings – Modified version for use in Jersey, is the document that was prepared in 2019 to define the methodology for the Jersey version of the Standard Assessment Procedure.  It is the basis of the rdSAP tool.  It is periodically reviewed and updated to take account of specific changes such as fuel price fluctuations.  Please see the attached document.

Jersey RdSAP V1.2.1_approved.pdf

In addition, the use of the SAP and rdSAP tools are supported by a conventions document which provides further guidance and instruction on the methodology and process of producing EPCs.  This is available in the following link:

​SAP 2012 Conventions.pdf (bregroup.com) ​