Energy efficiency of microwave and electric kettle for heating water Has anyone measured the watts used to heat a mug of water in a microwave instead of in electric kettle into cold mug?
I have always considered that an electric kettle wastes energy when heating enough water for a mug of coffee or tea. Enough water must be added to cover heating element and often surplus is left with hot element.Instead I have an old  tupperware mug with two finger handle for safe use. The correct amount of water is added. The heating is watched until it boils and then used immediately door opened.
According to the folks over at treehugger.com, they did a study to measure which of the stove, the microwave or the kettle was the more efficient in terms of use of electricity to heat a standard mug of water. This doesn't directly answer your question as you are unlikely to boil 350 ml (standard cup is 250 ml) of water in your kettle or on the stove, but it gets the point across.

A summary of their findings is that it took the electric stove 0.11 kWh, the microwave (900 W) 0.07 kWh, and the kettle 0.04 kWh. This indicates that in terms of energy usage the stove is 30.5% efficient, the microwave is 47% and the kettle is 81%. 

The clear winner is the kettle!