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.