What do we really mean by Energy Efficiency and Energy Performance?
What do we really mean by energy efficiency and energy performance?
Energy efficiency is a widely used term that suffers from issues of definition. This can be a cause of confusion. In a strictly technical definition, energy efficiency is simply useful energy output over energy input for any energy conversion device. For instance in an internal combustion engine, the energy output will be the rotational energy at the drive-shaft whilst the energy input will be the chemical energy contained within the fuel. For a power station, the efficiency will be useful electricity out divided by the energy content of the fuel input. However, this technical definition is not what is usually meant by energy efficiency.
Energy is used in two types of systems:
- Conversion devices such as internal combustion engines, light bulbs or power stations.
- Passive systems such as buildings where useful energy is degraded to low-grade heat in return for providing useful services such as thermal comfort.
We commonly use the term “energy efficiency” to cover both of these situations. In the case of passive systems it is technically incorrect as there is no (useful) energy output from a passive system such as a building, only useful services such as comfort. Here, a more accurate term than energy efficiency is energy performance. So the all-encompassing term ‘energy efficiency’ really incorporates two concepts:
- Technical energy efficiency (useful energy out/energy in) for conversion devices.
- Energy performance (energy in/useful output) for passive devices and systems.
Typical measures of energy performance include liters of fuel per 100 kilometers for vehicles, kWh per square meter for buildings to produce a certain temperature for a certain period of time, or kWh per 1,000 units of production in a factory.
For practical purposes energy efficiency can be defined as follows: “Energy efficiency is measured as the ratio between the useful output of the end-use service and the associated energy input. In other words, it is the relationship between how much energy is needed to power a technology (for example, a light bulb, boiler, or motor) and the end-use service (for example, lighting, space heating, or motor power) that the technology provides”.
Improving energy efficiency or reducing energy input for a given output is a process of technical and/or behavioral change that is driven by technological, financial, management, social and policy drivers and constraints.
The fundamental energy efficiencies of all technologies tend to improve over time due to improvements to existing technologies and the invention of new technologies.