Energy is what gets useful things done (“does work” in physics jargon). So converting it and moving it about is something people will pay for.

That’s why power plants generally make their money from selling energy (MWh) – even though we call them power plants rather than energy plants. And why your electricity bill is based on buying electrical energy; then using this in devices that convert it to heat up your dinner, light your room, play music and so on.

It’s crucial to recognise the distinction between energy (MWh) and power (MW).

The latter measures the rate at which energy is being output (from a power plant) or used (in a microwave oven, fridge etc.). Power can fluctuate instantaneously between a maximum (e.g. the capacity of a power plant or power rating of an end-use device) and zero: when you turn the lights on and off for example. Whereas energy accrues over time (MWh = MW x h): leave the lights on and your electricity bill gradually adds up.

Confusingly, there are many, many different units of energy (e.g. kWh, Btu, Joules, calories, eV, boe…) depending on the energy form, usage context and scale that they are being used for.