How primary energy is measured has changed across our charts

Energy can be measured at different stages of its journey from source to use, which can lead to confusion when comparing data. The four main stages are primary energy (raw energy inputs like coal or oil before any conversion), secondary energy (converted energy like electricity), final energy (energy delivered to consumers, e.g., gasoline in a car tank), and useful energy (energy actually used, such as light from a bulb). For example, burning coal in a power plant converts only about one-third of its energy into electricity, while the rest is lost as heat. This means 1 kilowatt-hour (kWh) of electricity from coal requires 3 kWh of primary energy input. The article focuses on primary energy, which is the starting point for most energy data and is used to compare energy sources like fossil fuels, solar, and nuclear power.

The substitution method was previously used by the Energy Institute to calculate primary energy for non-fossil fuel sources like solar and wind. This method converted renewable energy generation into its equivalent fossil fuel input by dividing the electricity output by the efficiency of a typical thermal power plant (around 38% to 41%). For instance, 100 terawatt-hours (TWh) of solar electricity would be counted as 250 TWh of primary energy under this method. The goal was to account for the inefficiency of fossil fuels, but it inflated the contribution of renewables in primary energy totals. This approach made fossil fuels appear to contribute less to the energy mix than they actually did, as their raw energy content was measured directly without adjustment.

The Energy Institute has switched to the physical energy content method for measuring primary energy, which reflects the actual energy supplied to the system without adjusting renewables to fossil fuel equivalents. Under this method, 100 TWh of solar electricity is counted as 100 TWh of primary energy, rather than 250 TWh. Fossil fuels are still measured by their raw calorific value, including the energy lost as waste heat. Nuclear power is treated differently because nuclear plants operate like thermal plants, converting heat into electricity with an average efficiency of around 33%. Thus, 100 TWh of nuclear electricity generation is counted as approximately 300 TWh of primary energy input. This change applies consistently across historical data, not just recent years.

The shift to the physical energy content method has significant implications for energy data. Primary energy from solar, wind, and hydropower decreases because it is no longer inflated to match fossil fuel equivalents. In contrast, primary energy from nuclear and geothermal increases slightly due to their lower thermal efficiency factors. The share of fossil fuels in the primary energy mix also increases under this method, primarily because the contribution of renewables appears smaller. For example, coal remains counted as 300 TWh for 100 TWh of electricity output under both methods, but solar drops from 250 TWh to 100 TWh, and nuclear rises from 250 TWh to about 300 TWh. This change is purely an accounting adjustment and does not reflect any physical changes in the energy system.