I boil a mugful of water in a kettle as and when required (boiling water is already off the boil as soon as you pour it into a thermos flask, so isn't much good for making tea, and it doesn't stay hot for that long if you keep opening it either.)
Microwaving a mugful of water is quite efficient, but can be hit and miss (either not long enough, or the possibility of superheating the water which can be dangerous as it can erupt when you move it from the microwave.)