Delivered energy can lead to net zero. Primary energy leads to confusion.
The Part L consultation has raised some old debates about using Primary Energy to measure the performance of buildings. In responding to the consultation, we dug into the numbers to make better sense of whether primary energy or delivered energy is best suited to deliver net zero.
Historically, the use of primary energy has been supported as a straightforward way of accounting for the upstream impacts of energy production beyond the walls of a building or system. The numbers suggest it is probably just simpler and more accurate to focus on delivered energy, combined with cost and CO₂ conversion factors.
Do you know anyone who is able to explain clearly how primary energy is actually calculated? Have you noticed how resistant to change many primary energy numbers have been, even as the electricity generation mix has rapidly changed? The answers to these questions should raise alarm bells. When 'clever' metrics such as primary energy are incomprehensible or incorrect, and no one notices, it can lead to perverse outcomes.
Delivered energy is simple. It is the metered energy that a building receives. It can be used to calculate energy expenditure or CO₂ emissions when combined with cost and emission data that is widely available, regularly updated, and well understood. These calculations are intuitive. Delivered energy can be estimated at design stage and can be measured once a building is occupied just by taking meter readings.
Primary energy is complex. A 2016 paper by Fraunhofer ISI evaluated different methods for calculating primary energy and concluded that none were clearly better than the others. Large variations in calculated primary energy were reported between the different methods. This means primary energy factors between organisations are often not comparable.
The primary energy factor for UK electricity is reducing. By 2030, it is projected to fall close to 1. As shown in the graph, even the 2020-2025 average is so similar to the primary energy factors for other fuels in SAP 10.1 that it fails to provide any meaningful insight. The proposed primary energy factor for electricity in SAP 10.1 will be out of date by the time it comes into effect.
Average primary energy between 2020 and 2025 calculated for UK electricity using National Grid 2 degree compliant scenarios, compared with proposed primary energy factors for other fuels in SAP 10.1
No consensus seems to exist on what primary energy actually is and how it should be calculated. Using nuclear as an example, it has a thermal efficiency around 30%, therefore could have a primary energy factor as high as 3. Some organisations use a value of 1, as it does not consume fossil fuel and result in 'losses' in a traditional sense.
It is not clear what the benefits of using primary energy are in terms of driving design decisions that result in genuinely low carbon buildings. Using of out of date factors for electricity risks encouraging heating with fossil fuels, as has been the case with the carbon factors in SAP for many years.
In summary, delivered energy is intuititive. It is simple to predict and it is already measured in every metered building. It can be used to calculate cost and CO₂ emissions using easily accessible data that is reliable and regularly updated. As the primary energy factor for electricity converges toward a value of 1.0, it appears to be becoming an academic figure with little real world application.