No Green Sheep

I was recently informed that boiling water in a kettle on the hob requires less energy than boiling water in an electric kettle. I wasn’t convinced as the informer had no numbers to back it up. So, I’ve done a little research and found that it is actually true.

The Numbers

The sources I have come across ^{[3, 4]} indicate that ~30% less energy is required to heat on the hob in a kettle. However, as the output of the hob increases the wasted energy also increases – and the electric kettle starts to win.

Power stations often operate with an efficiency of less than 40% ^{[1, 2]}. Heating by using gas directly is roughly 55% efficient (meaning that 55% of the energy spent goes into heating the water).

The Reasoning

When you burn gas to heat a kettle you are performing a simple one-step process. When you use electricity to heat a kettle you are taking part in a complex multi-step process: burn fuel, change thermal energy to kinetic energy, change kinetic energy to electrical energy, change electrical energy to heat energy. Inevitably inefficiencies creep in and add up.

But wait, there’s more…

Before you all rush out to buy kettles for your gas hobs there a several things to bear in mind:

1. Gas is a natural resource – we will run out one day
2. Your electricity may be coming from an eco-friendly source – if you use electricity from wind/solar then you are probably being more green than using gas
3. Most modern electric kettles turn off once the water is boiled – most gas hobs do not, so you might use more energy because you don’t turn the hob off as soon as the water boils
4. The efficiency of electricity will hopefully increase in the future
5. If it is winter, the wasted heat of a gas kettle goes into keeping your house cosy and warm
6. There is an increase in the risk of fires if you heat water using the gas hob.

Conclusion – Gas or Electric?

It depends.

If you tend to turn the kettle on and come back in a few minutes when it has boiled then you are probably better off with an electric kettle as it won’t keep on heating the water once it is boiled.

If you are cooking with the water you are probably better off using the hob – you are only going to be heating the food anyway.

So, before you boil a kettle – take a second to think about how you are using it and whether you are going to stay next to the kettle then pick the most appropriate method!

References

1. Why thermal power plants have a relatively low efficiency
2. Energy Efficiency in Power Plants
3. Which Kettle to Choose: Gas or Electric
4. How to Boil Water