What is the Primary Energy metric?
The Primary Energy metric is designed to give a more complete picture of the total energy used to power a building. In addition to the energy demand from the building itself, it also considers all of the energy used in upstream activities needed to get power to the property. This might include planting and cultivation of biofuels, extraction, refining and transportation.
Primary energy use is a measure of the energy regulated by the energy efficiency requirements of the Building Regulations, such as lighting, heating and hot water. The calculation takes account of efficiencies and energy uses such as:
• the efficiency of the property’s heating system;
• power station efficiency for electricity; and
• the energy used to produce the fuel and deliver it to the property.
“Primary Energy” simply means energy from a natural source which has not undergone any transformation or conversion process (such as fossil fuels or renewables). The energy demand for a property can be converted into Primary Energy by applying Primary Energy Factors to the different fuel sources.
For example, in SAP 10.1 (which will be replaced by 10.2 when it goes live), mains gas is given a PEF of 1.130. This means every unit of gas used in a property is equivalent to 1.130 units of Primary Energy. Each fuel type has its own PEF based on the energy typically required in its upstream activities.
Why is Primary Energy replacing CO2 within Part L?
Over the next few years, our buildings are expected to become increasingly decarbonised as grid energy switches to renewable sources and traditional gas boilers are phased out in favour of electrical heating methods such as air or ground source heat pumps.
Learn more about net zero buildings and energy decarbonisation
As this process continues, carbon emissions will become a less useful measure of how responsibly a building uses energy. This could lead to buildings which have low CO2 emissions but that are still wasteful and expensive to heat and power. By making Primary Energy the principal metric, retaining CO2 emissions as a secondary metric with heat demands also addressed through an improved building fabric (fabric backstops or fabric energy efficiency standard)it should be possible to ensure buildings are energy efficient and have a low carbon demand.
How is Primary Energy calculated for Part L?
As mentioned, Primary Energy Factors (PEFs) have been pre-calculated (within SAP) for each common fuel type based on upstream production activities needed to get them to market, using projections for 2020 to 2024 (These will be updated for the 2025 Future Homes Standard).
A property’s energy demand for each use (space heating, lighting etc.) is calculated and these are multiplied by the PEF of the fuel type used for each. By adding these together, we get a total Primary Energy demand for the property.
For example, Primary Energy demand for heating is calculated through the following equation:
(property energy demand/ efficiency of heating technology) x PEF
Let’s assume the property is heated with 100% efficient electric panel heaters with an overall heating demand of 10,000 kWh. The fuel factor for electricity is 1.501 so Primary Energy demand is calculated as follows:
(10,000 kWh / 1) x 1.501 = 15,010 kWh
Importantly, the Primary Energy calculation allows any energy generated by onsite renewable technologies, such as photovoltaic (PV) panels, to be subtracted from the overall energy demand. For example, if we added a PV array generating 1,500 kWh specifically for use within the property described above, then the calculation would be changed as follows:
([10,000 kWh – 1,500 kWh]/1) x 1.501 = 12,759 kWh
It’s important to note however that whilst renewable technologies can perform well under primary energy metrics, especially when installed on-site (less transport losses), costs can still be higher than necessary if the energy demand is not also addressed, as larger than necessary systems may need to be installed to compensate for poorer building fabrics. Ultimately reducing energy demand across the building stock means that less energy generation is needed in the first place and a more efficient fabric also means that services and systems can be more appropriately sized, so as to run more efficiently.