Which is Greener to Use

An electric kettle is often used to heat the small amounts of water for tea and coffee, because they are convenient. However, in earlier times people heated water using a kettle on the gas hob or solid fuel stoves, and for most domestic heating applications gas is cheaper. It is usually considerably cheaper (about 33% in the UK) per kilowatt-hour, and as an added advantage has a lower carbon footprint (200g/kWh for gas against about 500g/kWh for the UK Ref 1).
It Is Cheaper, Faster and Releases Less CO2 to Boil Water With an Electric Kettle
Surprisingly, it is in fact cheaper to boil water with an electric kettle. The reason for this is that an electric kettle is much for efficient at transferring energy to the water than using a gas hob. The increased efficiency of the electric kettle outweigh all the other advantages that favour gas.
How the Efficiency of an Electric Kettle Was Compared with a Gas Kettle
For both cases, 1 UK pint of water (0.568 litres) was heated to boiling point with the water starting from 12 C. One pint of water is a typical amount needed for two mugs of tea or drip filter coffee. A mug is typically a third of a pint but some excess water is need either for the filter or grounds, or remaining in the teabags.
In an ideal world it would take 0.568 * (100-12) * 4187 Joules of energy to boil the kettle, this is established by multiplying the volume by the rise in temperature and the specific heat capacity of water, resulting in about 209 kJ. Expressed in more familiar units of electrical energy this is 0.058 kWh.
Electric Kettle Efficiency and Speed Results
Boiling a pint of water (0.568l) starting at 12C using a Krups 2.4kW electric kettle of 1.6l capacity gave the following

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