I would like to know how to calculate the capacity of generator when a given water tank with a volume of 1000m³ capacity generates 2250KWH. Please advise.
Since the rating of a generator is expressed in terms of power, with a base of watts, and you have specified the capacity of the tank in volume and energy (base unit joules), there is one thing missing:
power = energy / time
watts = joules / seconds
So to answer your question you need to specify how many seconds it would take to empty the tank as well. The clue is in the "H" part of kWH.