The terawatt hour (TW·h) is a unit for measuring energy. It corresponds to 1,000,000,000 kW·h (kilowatt hours). It is the amount of energy that would be produced by a 1,000,000 MW generator over a period of one hour, or a 114 MW generator over a period of approximately one year.
The terawatt hour is commonly used for large amounts of electrical energy, since it may be easier to understand in a practical context than the proper SI unit for energy, the joule, which is a watt second (W·s).
1 TW·h = 1,000,000,000 kW·h = 3,600,000,000,000,000 J.