Just a few quick calculations: electric kettles usually hold between 1 and 2 litres, while old-style kettles may hold as much as 5 litres. To make a cup of tea, all you need is a quarter of a litre. Filling your one-and-a-half litre kettle to the brim can make tea for you and five of your friends. Do you really need to fill it right up and waste electricity or gas to bring the water to 100 degrees?
It’s much better to boil just the amount you’re going to drink and to serve it when the first bubbles appear, without waiting for the switch to trip or for the kettle to whistle.