Today, an experimental post. I was wondering what is the most efficient way to boil water, using a kettle or boiling on the gas hob? In order to determine this, I decided to knuckle down and experiment (for those of you who know me, you will realise this is somewhat out of character).
In order to determine how much energy it takes to boil half a liter of water, I turned the heating off and took a gas reading, including the little wheel reading that measures how many cubic feet of gas I have been using. I put the water in a non stick pot (lid on) and put it on the medium hob at max power. It took 5.25 cubic feet of natural gas to boil half a liter. Converting cubic feet of natural gas to Joules is not straight forward. In principle I would need to density of the gas, which might depend on atmospheric pressure and temperature etc. and also I would need to know the exact type of gas, it might in fact not be 100% methane. Luckily I found a website that does it for me. So to 1 significant figure, it takes 6 MJ to boil half a liter of water.
To work out how much electricity it takes to boil half a liter, I simply timed the kettle. It took 103 seconds. The kettle claims it is rated at 2000/2400 W. This implies that (to one significant figure) it takes just 0.2 MJ using the kettle. This is actually a very small amount of energy. In fact, it is possible to determine the minimum possible amount energy required to raise half a liter of water from room temperature (20 degrees) to boiling (100 degrees). The specific heat of water is 4.2 J/g, so I deduce that the minimum amount of energy it takes to boil half a liter is 0.17 MJ. The kettle is approximately 85% efficient!
Comments on the validity of this approach. I wish I had a thermometer to determine the exact temperature of the water, rather than relying on when I think the water is boiling in the pot. I also wished I had a high voltage ammetre to measure the actual power usage of kettle.
The conclusion of all this is that the kettle is massively more efficient (almost 20 times more efficient) than the gas on the hob. I believe that even if we allow for the fact that electricity production is not 100% efficient the kettle wins. So long as you only boil as much as you need!