GWh

In electricity, the watt (W) is the unit of power of a system in which a current of 1 ampere flows across a potential difference of 1 volt. It's an average instantaneous power. Nuclear reactors like those in Belgium have around 1,000 MW of power (1,000 megawatts, which is a billion watts or a gigawatt, GW).
In one year, an energy source with a power of 1MW will produce a number of MWh (megawatt hours) of energy equal to 8,760, or the number of hours in a year.
But Wh (watt hours) must not be confused with W/h (watts per hour), which indicates the variation in power per unit of time.