hi, when I did my city and guilds electrical course, we did a comparison of boiling a kettle by electric and heating one on the gas. the outcome was: a kettle element heats only the water inside the kettle, all the energy is used, although the kettle gets hot on the outside. the gas hob, heats the kettle from below, but a great deal of the heat energy rises up the sides and into the atmosphere. heating the metal base of the kettle heats the water as a secondary effect. cost wise, the gas actually won as the cost of electricity is far greater than gas. carbon footprint-wise, electricity is the least pollutive . but, it was pointed out that in the winter, the wasted gas heat was also keeping the kitchen warm, so was not really being wasted.so, electric in the summer, gas in the winter.