A watt-year (Wyr) is the measure of energy output by one watt in one year. This unit almost always refers to an amount of electricity generated by a group of power plants or an entire electrical grid.
Far more commonly, larger units of Mega watt-year (MWyr) and Giga watt-years (GWyr) are used. These units are helpful when describing the output of a large power plant over the course of a year. For example a 650 MW CANDU reactor will put out 650 MW-years over the course of a year.
This unit is closely related to kilowatt-hour, another energy unit which is a power times an interval of time.