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The addition of a third compliance metric into the new Part L, known as Primary Energy (PE), will push developers further into adopting solutions which demonstrate a high standard of energy efficiency at an infrastructural scale.

Primary Energy is best understood as a more complete measure of the energy used in a building, taking into account energy ‘spent’ in the harvest and transport of fuel sources, as well as the efficiency of how it is distributed and ultimately used. This additional metric serves the purpose of protecting the electricity generation & distribution infrastructure from overload.

Importantly, the mains electricity PE factor is higher in new Part L than that of mains gas. This is a consequence of energy-intensive processes inherent in the extraction and refinement of nuclear and the remaining fossil fuels which are used to generate electricity, and the significant transmission losses on the electricity grid. Whilst the averaged carbon emissions associated with electricity generation are now lower than that of gas use, the energy used prior to this point remains much higher.

In practice, this means that direct electric heating & immersion hot water are discouraged. The use of these will require much greater enhancements to other areas of the specification for a building to achieve the required PE rating than if a heat pump (with a system efficiency of around 300%, compared to 100%) was used.