What do we really mean by ‘energy efficiency’?

There is still often confusion about what we really mean by energy efficiency. It is a phrase that comes with a lot of baggage and associations with doing less and sacrifice – the old idea of ‘energy conservation’. It even gets confused with renewable energy sometime which is strange given renewables are a means to generate energy and not a way to improve efficiency.

First of all ‘energy’ itself in the way that we use it today is a relatively modern and confusing term – we should really talk of fuel and electricity as they are very different and incompatible sources of services – you can’t put petrol in your laptop. Physicists and engineers also know that energy is always conserved in any process so the terms, ‘energy conservation’, ‘saving energy’ and even ‘consuming energy’ are technically incorrect.

The term ‘energy efficiency’ incorporates two concepts. The first is energy efficiency in its technical sense – useful energy out/energy in – which is usually reported as a percentage – and can only be applied to devices that convert one type of energy to another such as engines (chemical energy to motion), electric motors (electricity to motion) or light bulbs (electricity to light). The second concept is energy productivity, usually reported as energy in/useful output, which applies to passive devices such as buildings which convert energy into other services such as comfort. We are familiar with some everyday measurements of energy productivity, such as miles per gallon or litres per 100 kilometre for car fuel efficiency and others including energy input to a building per square metre to produce a certain temperature for a certain period of time; energy use per passenger mile for aircraft; or energy per one thousand tins of beans produced in a factory. So what we normally call ‘energy efficiency’ is really a combination of energy efficiency in its technical sense and energy productivity.

Just to add to the story, when we commonly talk about energy efficiency in a macro-sense we often mean a series of processes rather than a status at a single point in time. The energy efficiency of all technologies tends to improve over time because there is a basic human desire to spend less, invent new technologies and improve existing technologies. As well as the constant incremental technological (and behavioural) changes, there are the major paradigm-busting changes such as a compete change of an industrial process that work to improve energy efficiency over time.

So to sum up, the phrase ‘energy efficiency’ is a combination of technical energy efficiency and energy productivity, and it is also a process of continuous improvement.