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Andrew J Ball

On Construction Domestic Energy Assessor (OCDEA) for new-build home energy SAP ratings, providing On Construction Energy Performance Certificates (OCEPCs)

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SAP Ratings Explained / Introduction to SAP Calculations


This web page introduces SAP ratings / SAP calculations and seeks to explain the stages in the SAP calculation process for both the client and the assessor.


A SAP rating is the technical calculation that is required in order to produce a Predicted Energy Assessment (PEA) and an On Construction Energy Performance Certificate (OCEPC) both of which are reports that abstract information from the supporting SAP calculation.


A SAP calculation is simply a home energy rating that seeks to calculate a score between 1 to 100+ for the annual energy cost due to

  • the built structure of the home
  • its heating and hot water system
  • internal lighting
  • any renewable technologies used in the home.


The higher the number the lower the fuel running costs, with 100 representing zero energy cost. Dwellings with a rating in excess of 100 are net exporters of energy.


In simple terms SAP calculations allow comparison of the energy running costs of dwellings anywhere in the United Kingdom, in a similar manner to the well-known comparison of fuel economy in cars expressed in miles per gallon. This is achieved because the calculations are predominantly location independent and are based upon a notional standard occupancy that overcomes variations associated with physical location and the differing ways in which people utilise their homes.


A SAP calculation is normally a desk exercise. The assessor does not usually visit the dwelling being assessed.


The client submits drawings, plans and specifications of the development to the assessor.


Before commencement of any work the assessor provides the client with a formal quotation for the SAP rating work.


SAP Calculations involve the following 4 no. stages:-


1) Design - Draft stage

From the plans and drawings the assessor prepares summary numerical information such as the total floor area of the dwelling; the floor area of the lounge or living room; the areas of the heat loss floors, heat loss walls and heat loss roofs; dimensions of any external windows and doors; the heights of each storey.


From the specification provided the assessor calculates the thermal performance of building elements such as heat loss floors and heat loss walls. These are expressed as 'U' values - measures of the rate at which heat passes through the fabric of the building. The higher the 'U' value the greater the rate of heat loss.


The assessor uses a computer software program to input data for the SAP calculation. Data is entered relating to

  • The type of dwelling
  • Floors
  • Walls
  • Roofs
  • Openings (windows, doors, rooflights)
  • Ventilation
  • Main and secondary space heating
  • Hot water generation
  • Use of renewable technologies such as phovoltaics and solar water heating
  • Lighting

There are in excess of one hundred data items involved in the calculation.


The computer program determines whether the proposed dwelling will pass the Building Regulations with regards to the Conservation of Fuel and Power. The assessor is able to use the software to model different variations of the dwelling design.


The assessor reports back to the client.


The client validates the information provided by the assessor.


2) Design - Final stage

The client and the assessor agree a finalised version of the design, sometimes following several amendments to the initial design.


Data from the finalised design is input into the computer program.


The assessor provides reports that the client may need to submit to the appropriate building control body that will be monitoring building regulations compliance. For new dwellings this will include a Predicted Energy Assessment (PEA) - that provides a rating of energy performance based upon the specified design.


Sample PEA


3) As Built - Draft stage

The client continues with the build until completion.


For new dwellings an air pressure test will normally be required, and the client arranges for this to be undertaken by a suitably qualified operative.


The client reports back the results of the air pressure test to the assessor and also advises the assessor of any variations from the previously agreed Design - Final specification.


The assessor edits the SAP calculation to reflect the results of the air pressure test and any variations to the Design - Final specification. The software is used to check that the completed dwelling still meets the requirements of the building regulations with regards to the Conservation of Fuel and Power. If it does not the assessor recommends remedial action.


The client validates the information provided by the assessor.


For new build dwellings the assessor checks to ensure that the dwelling is registered on the government's central database register of national property addresses. If it is not the assessor arranges for the address record to be created.


4) As - Built Final stage

The assessor finalises the SAP calculation and creates the On Construction Energy Performance Certificate (OCEPC) - that provides a rating of energy performance based upon the dwelling as built. The OCEPC must, by law, be displayed in a new dwelling put up for sale on the open market.


Sample OCEPC


In addition there are other documents that may be required by the relevant Building Control Body such as the SAP Data Input Report and the SAP Worksheet Report.


The assessor provides the client with the reference of the OCEPC so that the client is able to obtain a copy from the government's central registry website at