A kettle is one of the most powerful things in the house, pulling around three kilowatts, and the average home boils it several times a day. Each boil is brief, so no single one costs much, but the habit repeated for years makes the kettle a worthwhile target, and the fix costs nothing.
Only boil what you need
The single biggest saving is to boil only the water you are about to use. Filling the kettle to the top for a single mug means heating several extra cups of water for no reason, every time. Measure your mug into the kettle, or use the cup markings on the gauge, and you cut the energy per brew straight away. Across a tea-drinking household over a year, that small discipline adds up more than people expect.
Descale for speed and efficiency
In a hard-water area, limescale furs up the element and makes the kettle slower and less efficient, since the scale gets in the way of heat reaching the water. Descaling regularly with a cheap descaler or white vinegar keeps the element clear so the kettle boils quickly and uses less energy doing it. A heavily scaled kettle is both slower and thirstier.
Eco kettles and the flask trick
So-called eco kettles, with clear gauges and low minimum fills, mostly just make it easier to boil less, which you can do for free by paying attention. Rapid-boil kettles save time rather than energy. A genuinely useful habit for big tea drinkers is to boil once and keep the rest hot in a vacuum flask, so a single boil serves several drinks across the morning rather than reboiling each time.
Kettle versus the alternatives
For boiling water, an electric kettle is actually efficient, putting nearly all its energy straight into the water, so heating a mugful on a gas hob or in a pan is usually slower and no cheaper. The kettle is the right tool; the waste is purely in overfilling it. Put your own usage through the cost calculator and the case for boiling less makes itself.