look out piano
Striker
My head hurtsIt's not straight forward.
If you're heating something substantial, like a full tin of soup or beans, a gas hob is cheapest, then electric hob, then microwave.
Gas is cheapest because, although it's only 50% efficient, gas is 25% of the cost of electricity, so you may use twice the energy, but it's still half the cost.
Electric hob is much more efficient, but you do need to heat the pan and hob, as well as the soup.
Microwave is less efficient because an 800W microwave oven is giving out 800W of microwave energy, but the oven will consume around 1200W of power to generate that 800W of microwaves.
For small items, like the 150ml of water in a small cup of tea, the microwave becomes cheaper than the hob because only the water gets heated in the microvave, there's no need to heat the pan as well, and, for small quantities, heating the pan becomes more significant than heating the contents.
The margins are quite small.
To boil 1 litre of water consumes about 0.1 units of electricity. At 27p/unit that's 2.7p per litre, or about 1p for your 400g tin of beans.
So, even at today's prices, you could heat your tin of beans for a penny on an electric hob, or a bit over a penny in the microwave.
It's better to concentrate on savings elsewhere.