Plug in: Energy Use
What is a Watt? What is kWh? It depends on who you ask.
Let’s take a myth-busting dive into the standard metric for building energy use.
We’ll all remember from school that a Watt is a unit of power, named after James Watt whose 1776 steam engine led to the Industrial Revolution and (somewhat ironically) the very mess we find ourselves in today. A kilowatt is 1000 Watts. A kilowatt hour is the power consumed by something over one hour. For building energy, it is often given as a ‘specific value’, which means it is relative to floor area. To understand the power over a full heating season, we need to assess a full year. This gives us the familiar kWh per metre squared per year, or kWh/m2/a.
So far, so logical.
But try comparing one building to another and you could quickly become unstuck. Building A may be quoted in kWh of Primary Energy. Building B is in kWh Primary Energy Renewable (PER). Building C is kWh Energy Use Intensity (EUI). Building D is kWh ‘in-use’, including regulated energy but excluding unregulated energy. Eh?
Despite using the same unit of measurement these all mean completely different things, which can lead to both intentionally misleading statements and genuine confusion. It is also a barrier to anyone without a PhD who is seeking to give or receive energy advice.
Just like scoring a goal in football is different to shooting a goal in netball or kicking a goal rugby. What’s the score? 6-4. Football?! No, Ice Hockey….
If you want to make decisions based on these figures, then you need to understand the difference.
In the UK, EPCs state Primary Energy. This is the energy needed upstream at the power station to produce the energy you need in your home, factoring in that some gets lost on the way. A big failing of the EPC system is using Primary Energy - a metric that consumers cannot relate to, and that designers cannot design to. Scotland is looking to change the terminology of Energy Efficiency Rating to Energy Cost Rating and add a new metric called Energy Use Rating to clarify this riddle for homeowners.
If the home used 100kWh of gas at the meter, it may need 123kWh of Primary Energy to start with. That’s why the government cares about Primary Energy and the cost of this. Each type of fuel has a Primary Energy Factor to allow a single kWh in the home to be multiplied up to the Primary Energy value depending on what fuel it was derived from. However, consumers care about Delivered Energy (or Final Energy) being the figure they see on their bills that reflects their actual usage. They have the power to influence this figure.
The Passivhaus Institute created the Primary Energy Renewable or PER for its kWh unit. This seeks to factor in the renewables that will feed into the national grid over the life of the building, so totals how much renewable energy will be needed to produce the Delivered Energy, including some losses.
The new kid on the block is Energy Use Intensity or EUI and is being championed by LETI and others as a more user-friendly unit. It is a total energy unit, so includes regulated energy (building systems) and unregulated energy (or plug loads). Like Delivered Energy it is based on actual metered usage but disregards how carbon intense the grid is now, anticipating a future when all grid power is carbon free.
EUI is both a unit that consumers can recognise and that designers can design to. LETI is recommending that it becomes the standard way to state kWh. A recent PHPP plug-ins allow designers to convert PER into EUI for greater transparency.
If you made it this far, well done! The main takeaway is to remember there are differences and beware of those blindly quoting A vs B. You might be comparing Netball and Polo.
