terawatt hour

A watt is a rate. It measures how fast energy is being produced or used at a given instant. A watt hour is an amount: one watt sustained for one hour. The difference matters because power and energy are constantly confused in reporting, and a terawatt hour is a unit of energy, not of power.

The prefix tera means a trillion, ten to the twelfth power, and is one of the metric prefixes adopted into the international system of units in 1960. The hour is not itself an SI unit, but it belongs to a small set of non-SI units accepted for use alongside them, defined as 3600 seconds. Put together, one terawatt hour is a trillion watt hours.

The scale is easier to feel by comparison. A single large power station might produce a few terawatt hours in a year, while a mid-sized European country gets through a few hundred. One further distinction is worth carrying: a figure in terawatt hours describes electricity actually generated over a period, which is not the same as the capacity that was installed and standing ready to generate it. A country can add a great deal of capacity and generate less than expected, or the reverse.