terawatt-hour

The terawatt-hour is electricity at national scale. Annual generation and consumption of whole countries are quoted in TWh: the world generates roughly 30,000 TWh of electricity a year, the United States about 4,200, and France about 500, of which around two thirds is nuclear. Any argument about whether an industry uses more electricity than a country is conducted in TWh per year.

The number worth memorising is 8.76: a gigawatt of capacity running flat out for a year produces 8.76 TWh, because a year is 8,760 hours. Real plants run below that ceiling, and the ratio of delivered TWh to the flat-out figure is the capacity factor, which is the quantity the wind and solar output solvers on this site turn on. A nuclear unit at 90 percent capacity factor delivers about 7.9 TWh per gigawatt-year; onshore wind at 35 percent, about 3.1.

The trap is treating the unit as a power. TWh per year is dimensionally a power, and a modest one: one TWh a year is a continuous draw of about 114 MW. A headline saying a country consumes 500 TWh almost always means per year, but the unit itself does not say so, and a stock of energy such as a reservoir and a flow of energy such as annual consumption both arrive in TWh with nothing but context to separate them.