In case anyone ever tries arguing with you that lower powered kettles are more efficient... Please apply the following simple bits of physics: convection, radiation, evaporation.
The longer the kettle spends getting from 30°C to 100°C, the more these sources of heat loss will come into effect, therefore low power kettles are better at heating the air, and less efficient in heating the water.
(The icon is for a nice efficient way of heating /lots/ of water.)
PS. Would someone like to test how hot a 25W fish tank heater (without a working thermostat) will actually get a typical fish tank? My guess is somewhere around 50°C, but I don't have a fish tank to test it these days.