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Primary, secondary, final, and useful energy: Why are there different ways of measuring energy?

There are four key ways of measuring energy. These metrics capture the transformations and losses that occur across the energy chain.

The differences between the first stage (‘primary energy’) and the last (‘useful energy’) can be very large. This means it’s important to be clear about which metric is being referred to when people speak about data on “energy”.

In this post I explain these four metrics.

Primary, secondary, final and useful energy: the four measures relate to the four stages of the energy chain

In the visualization, I’ve shown the four stages of the chain and provided examples of this pathway for different products.

Primary energy: Primary energy is the energy as it is available as resources – such as the fuels that are burnt in power plants –  before it has been transformed. This relates to the coal before it has been burned; the uranium; or the barrels of oil.

This is the most widely available statistic and very commonly used.

Secondary energy: When we convert primary energy into a transportable form we speak of secondary energy. For example, when we burn coal in a power plant to produce electricity, electricity is a form of secondary energy. Secondary energy includes liquid fuels (such as gasoline and diesel – which are refined oil), electricity, and heat.

Final energy: Once we’ve transported secondary energy to the consumer we have final energy. Final energy is what a consumer buys and receives, such as electricity in their home; heating; or petrol at the fuel pump.

Useful energy: This is the last step. It is the energy that goes towards the desired output of the end-use application. For a lightbulb, it’s the amount of light that is produced. For a car, it’s the amount of kinetic (movement) energy that is produced.

Four ways of measuring energy

Each stage of the energy chain results in losses

As the illustration above showed, at each stage of the energy chain, some energy is lost or wasted. The four metrics capture energy losses in the following ways.

Primary to secondary energy: the conversion of primary to secondary energy can be very inefficient. In thermal power plants – which convert fossil fuels, biomass or nuclear into electricity, up to two-thirds of the primary energy is wasted as heat. For every three units of energy we put in, you get just one unit of electricity out.

Because primary energy losses are particularly large for fossil fuels, their contribution to energy demand is much higher in primary energy terms compared to the other three ways of measuring energy. This is important to know because it can skew our perception of how much of a contribution low-carbon sources make: in primary energy terms they can appear smaller because they are diluted by the wasted energy that comes along with fossil fuel burning.

Secondary to final energy: we also lose energy in the process of delivering it to the consumer. This is called a ‘transmission and distribution’ loss. When we transport electricity from a power plant (secondary energy) to homes (final energy), for example, we lose some while transmitting it through power lines.

Final to useful energy: no appliance is completely efficient in providing only the desired output that we want. 

For a lightbulb, the useful energy – what we want – is the light. But bulbs also produce some heat. The useful energy from cars is movement. But engines also produce heat and noise.

Any energy that is not used specifically for the desired use of an appliance is waste.

The energy we need as the end-user is often a small fraction of what goes into the top of the system. We see this in the schematic.

The world produces a lot of energy, and most of it is lost along the way. The four different measures capture the energy that is available at different stages along this chain.

Primary energy losses

Looking at the four stages of the energy chain can help us to identify inefficiencies

There are valid reasons to look at any of the energy metrics individually. But we need to be aware of what the differences mean, because they can be very large. 

One good reason why we want to study each of these measures is that they tell us about what options we have to reduce inefficiencies in our energy system.

A very efficient energy system is one in which primary and useful energy use are very similar. We are currently far away from such a system. The inefficiency losses are often large.

We can reduce losses from primary to secondary energy by transitioning away from fossil fuels because the losses for these sources are particularly high. This reduces the amount of heat lost from converting these raw fuels into a usable form, such as electricity.

We can reduce losses from secondary to final energy by reducing transmission and distribution losses. This means creating more effective energy delivery networks, such as better-integrated electricity grids.

We can reduce losses from final to usable energy by improving the efficiency of final appliances. Lightbulbs today are much more efficient than they used to be – they convert more energy into light, and less into heat. Car engines have become more efficient. This is especially true for electric vehicles; they convert much more final energy into kinetic energy, and lose much less as heat and noise. On average they lose just 15% to 20%, compared to losses between 64% to 75% in a gasoline engine.

Looking at the energy chain as a whole, we can identify where the largest losses occur, and where interventions can have the biggest impact.

Keep reading at Our World in Data

Acknowledgments: I would like to thank Max Roser for useful feedback and suggestions on this article.