Energy Units: What are we talking about? Some definitions

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Published on 11/11/2017

Everyone can read or hear in the media various units (kW, MW, GW, TWh …), quoted and manipulated sometimes without knowing what we’re talking about … Some definitions, then, to understand what we are told about energy…


Expressed in W (watts), kW (thousand watts), MW (1000 kW), GW (1 million kW), this unit reflects the theoretical maximum capacity of a power generation device. In other words, it’s the maximum energy it can provide per second. So this unit does not include ideas of duration of use, nor of “piloting” the energy generated according to needs. It is often called “installed capacity”.

Energy generated

Here, we mean energy that can be measured on an instantaneous or average basis (daily, monthly, annually) at the output of an energy-producing device, as well as consumption by end-users.

On our scale, it is expressed in kilowatt-hours, or kWh (what we pay on our electric bill – the order of magnitude of consumption for a 75 m2 apartment that is properly insulated, with electric space heating, is 7 000 to 8,000 kWh per year). We will also talk about megawatt-hours or MWh (thousands of kWh), for example on the scale of a generation device, gigawatt-hours or GWh (1 million kWh) for large installations, and terawatt-hours orTWh (1 billion kWh) on a scale of a country.

These two concepts are not to be confused!

The demand for energy varies with time, and significantly depending on the season, especially in temperate countries such as France. The production capacities will therefore be mobilized in a variable way throughout the year, the electricity network ensuring the “piloting” of the various means of production to guarantee to everyone, whatever his location and resources, the basic power supply he requires. We therefore need facilities that can guarantee the necessary production, on the one hand, and modulate it according to needs, on the other hand.

And in this sense, not all means of production are equivalent! In particular, one cannot talk about installed power alone! For example, a fossil-fired power plant or nuclear power station of 1 GW power will be able to produce about 8 TWh of energy over a year while running at full power, counting a month off for maintenance. A photovoltaic solar power plant with the same installed capacity, based on a potential utilization rate of 14% (French average), will be able to produce 1.2 TWh over the whole year, i.e. more than 6 times less. Moreover, in winter beginning around 5 p.m. until sunrise, the production of that plant will be 0, whatever its installed power.

Attention must therefore be paid to comparisons between the installed power of intermittent generation sources – which are not controllable, because one does not control the wind, nor the sun – and the installed power of controllable power plants that are able to produce continuously!

What counts ultimately is the energy produced, which must adapt in real time and according to the seasons to the changing needs of all users – who are also the ratepayers