In electrical power production and sale, the standard unit of energy is the kilowatt-hour (kWh). That is the energy produced or consumed over a period of one hour by a power of one kilowatt. If your electricity bill says your house used 70 “units” of electricity last month, it means it used 70 kilowatt-hours of energy.
Hydro generation typically supplies hundreds or thousands of houses, so we typically measure generation as megawatt-hours (MWh) or gigawatt hours (GWh)