[Piwoslaw]

Did a test, here's what I got:
I boiled 500ml of room temperature (18*C) tap water in an electric kettle, in the same kettle while holding a fleece blanket around it, in the microwave, and in a small pot on the stove.
Kettle: 1960 watts, 3 minutes, used 58Wh.
Kettle with blankie: 1960 watts, 3 minutes, used 57Wh.
Microwave: 1360 watts, 6 minutes, used 109Wh.
Stove: 6 minutes.

Comments:
Kettle with blankie: After the first boil I poured out the water and waited 20 minutes for the kettle to cool down. During the second boil I held a blanket around the sides of the kettle. As expected didn't really help. After the 3 minutes, the sides of the kettle were barely luke warm.
Microwave: The water was in a Pyrex glass cup. The 1360 watts includes extras like: dispay (3W), light (40W), turntable and fan (?W).
Stove: Read in the stove's instruction that the burner is 58% efficient. Found a way to open the box with my meter, but it didn't budge during the test, so I either used less than the meter's resolution of 0.0001 cubic meters, or the stream was too weak to overcome the meter's internal resistance. Maybe a test with boiling 5 or 10 liters would give something?

The outcome is clear: just keep boiling in the electric jug, as I've been doing. I was kind of hoping the microwave would be more efficient for a small amount of water, but it appears that even without the parasitic loads it won't work. Maybe a newer unit would be more efficient?