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.
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!
Have you considered a kettle with an embedded element? I don't think I've seen a 'floating' element kettle in 20 years. Jug kettles [tall & thin] are capable of boiling half a cup of water in about 30 seconds.
You can do the math [approximately] yourself on the equipment you currently have.
Assume a kettle is 2kW a microwave 1kW
As you can't put one cupful in your kettle, you'll have to work with what you can.
Put minimal water in kettle, boil & time it.
Put one cup in the microwave, boil & time it.
If the microwave takes more than twice the time of the kettle, the kettle is more efficient.
This would be more accurate once you know the precise wattage of both devices & far more energy-efficient with a modern jug kettle.
Note the amount of heat energy getting into the water is not necessarily the entire output of the heating device, so timing it & multiplying by the wattage is the only simple & effective way to tell how much energy was used.
Also note that convection is less effective in a microwave, so you will have to stir half-way. Water can boil at the top yet still only be 50° at the bottom of a cup.