The brand of kettle is needed to determine the exact amount of electricity used, but most kettles have an average wattage of 2 or 3 kilowatts or kW. If a 2 kW kettle is used for 2 hours then it has used 4 kWh.
To help conserve energy usage of a kettle make sure to only heat up what is needed. If there are three people wanting a cup of tea each, only heat up 3 cups of water instead of the whole kettle. Heating up only 3 cups takes less time than it would to heat the whole kettle. Try to buy a kettle with an automatic shut off feature, so when the water is done boiling it cuts off and does not use up more energy.
The use of an "eco kettle" has been said to conserve energy and allows the user to have the option to adjust the temperature. If the kettle is only going to be used for the occasional cup of tea, get a smaller kettle. If the kettle is needed for multiple functions, for instance making a pitcher of tea, instant soups and other purposes, get a kettle that will hold a lot of water.