It should be noted that the OpenBuildings PartL program does not actually perform the PartL calculation - this is performed by SBEM.
The processes involved in the creation of Energy Performance Certificates and Part L Compliance documents originate from the European Energy Performance of Buildings Directive, which set out requirements in relation to measuring energy usage in buildings.
To achieve this, the Department of Communities and Local Government devised the National Calculation Methodology and BRE were entrusted with developing suitable software.
"The NCM allows the actual calculation to be carried out either by an approved simulation software or by a simplified tool based on a set of CEN standards. That tool has been developed for DCLG by BRE and is called SBEM - Simplified Building Energy Model"
OpenBuildings Energy Simulator PartL program is an interface into the SBEM calculation engine. The PartL program generates a correctly formatted input file that is passed into the SBEM calculation engine. As such, Bentley Systems do not have access to source code or the calculation methods used by the SBEM calculation engine.
User should always consult the appropriate technical manuals for the most up to date information.
Useful guidance and documentation on the above can be found at The BRE website , including SBEM technical manuals, NCM Modelling guide, and the NCM databases