When I’ve boiled up the kettle, using the minimum water, I have an almost empty kettle. I quickly pour in a mugful of water. This helps to catch the heat left in the element which would otherwise be lost to the atmosphere.
You’ll save a few joules when you make the next tea of coffee.
I despair at folks who put a quart of water in the kettle just to make a couple of cups. Pointlessly boiling that extra pint takes a pile of extra energy and gains nothing. It takes longer and will hit you in the pocket when your bill comes.
I keep a mug by the kettle, when I want to make a cuppa I just fill the mug with water use this to fill the kettle. If I’m just making one cup then covering the element will suffice.
Once you’ve poured the water, there’s another little trick!