How primary energy is measured has changed across our charts

Reporting by Our World in Data

Some of the most popular charts on Our World in Data are about energy production and consumption: how much energy countries produce, how they compare per person, and what sources this energy mix comes from.

But when people talk about “energy”, they’re not always talking about the same thing. There are different ways of measuring energy, from the very top of the chain — for example, coal going into a power plant — to the final link in the chain — the light that eventually comes out of a lightbulb.

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

In a previous article, I walked through the four different ways of measuring energy. I’ve included the main visualization below.

On Our World in Data, we present data on both primary energy and secondary energy in the form of electricity generation. We would like to present final and useful energy too, but international datasets on these metrics are, unfortunately, not openly available.

But even when it comes to primary energy, there are differences in how this is calculated. Recently, the methodology used by our main data source changed. In this article, I want to explain this change and how it affects our charts on this topic.

The main difference in the calculation of primary energy comes down to how we account for the energy wasted when we burn fossil fuels.

If you put coal into a power plant, around two-thirds of its energy value is wasted as heat, and only one-third is converted into electricity. For gas, roughly half is wasted, and half is converted to electricity. So to get 1 kilowatt-hour (kWh) of electricity out, you need to put in the equivalent of 3 kWh of coal and 2 kWh of gas. I’ve sketched this in the diagram below.

This is not the case with non-combustible sources, such as solar, wind, and hydropower: they just deliver electricity. We just see 1 kWh of electricity output; there’s no upstream fuel input or waste along the way.

That means that 1 kWh of electricity from coal, 1 kWh from gas, and 1 kWh from solar will look very different when measured in primary energy terms. It would be equivalent to 3 kWh of coal, 2 kWh of gas, and 1 kWh of solar. Expressed as a share of primary energy, coal would make up half of our energy mix, gas one-third, and solar one-sixth. Even though the electricity we get out — and can actually use — is split evenly between coal, gas, and solar, each makes up one-third.

Our main data source for primary energy is the Energy Institute’s Statistical Review of World Energy.1

It historically presented data on primary energy using the substitution method.

This method attempts to account for the inefficiency of fossil fuels by converting renewable and nuclear energy into their “fossil equivalents”. In other words, it asks: if this electricity had been generated from fossil fuels instead, how much fuel would have been used?

To get this figure, it divides renewable and nuclear electricity generation by a typical thermal power plant efficiency factor, which is around 38% to 41%. So, if a country generates 100 terawatt-hours (TWh) of electricity from solar, that amount is divided by 40% to get a “fossil equivalent” of 250 TWh.