Which kettle is more energy efficient?
Boiling water is an essential pre-requisite for the preparation of tea – and as such it is an activity central to the cultural life in England. For example, six cups of tea were drunk while preparing this article. Boiling water is also an energy intensive activity and so for all the usual reasons, it’s a good idea to boil water in an efficient manner. But should one use an electric kettle or a gas kettle? This was the subject of my first ever blog posting back on 1st January 2008 – and because that blog is no longer available, I thought it worthwhile to re-post and re-visit the problem now.
Initially, I was confident that using a gas kettle would save energy compared with an electric kettle. This is because electricity is generated at power stations which have an overall efficiency which is often less than 40%. That’s a guideline figure for any thermally-generated electricity (coal, gas or nuclear). It means that roughly 60% of the energy content of the fuel is lost before the electricity leaves the power station. Some gas generation using CCGT technology is more efficient: up to 60% in the best cases but relatively little electricity is generated this way. Additionally, typically 10% of the energy which leaves the power station is lost in transmission to my home. So only perhaps 36% of the original energy content of the fuel is fed into my kettle element.
Since raw methane is distributed directly to my kitchen, all a gas kettle has to do is capture a bit more than one third of the chemical energy and I would be better off than using electricity. So I was confident that by using a gas kettle I would be making an environmentally friendly decision.
Was I right? I compared the time boil 1 litre of water with the time expected if the device used the energy supplied to it in the kitchen with 100% efficiency. I then applied a ‘Network Efficiency’ factor.
The Electric Kettle (2 kW) took 210 seconds to boil 1 litre compared with an expected time of 188 seconds, so the ‘in-kitchen’ efficiency was 90%.
The Gas kettle had three burners:
- The 1 kW Burner took 680 seconds to boil 1 litre compared with an expected time of 380 seconds, so the ‘in-kitchen’ efficiency was 56%.
- The 1.75 kW Burner took 450 seconds to boil 1 litre compared with an expected time of 210 seconds, so the ‘in-kitchen’ efficiency was 48%.
- The 3 kW Burner took 380 seconds to boil 1 litre compared with an expected time of 127 seconds, so the ‘in-kitchen’ efficiency was 33%.
The Microwave Oven (800 W) took 720 seconds to boil 1 litre compared with an expected time of 300 seconds, so the ‘in-kitchen’ efficiency was 66%.
And the results?
Result #1 Heating water using a gas kettle is a more efficient use of fuel, but only at low gas settings. And even at low settings, a gas kettle wastes 50% of the energy supplied to it. Using higher gas settings wastes an even larger fraction, up to 67%!
Result #2 Heating water using an electric kettle is the fastest way to heat water, being roughly twice as fast as the high-power gas burner – and with very similar efficiency.
Result #3 Using a microwave to heat water was definitely the least efficient of any of the methods – but this result may change when heating small quantities of water.
Overall the data indicate that gas kettles are amazingly wasteful. In subsequent articles I will be looking to see if they really are as bad as they seem.
The efficiency of different ways of heating water. The blue dots show the efficiency with which fuel supplied to the kitchen is converted to heat energy in the water. The red dots show the efficiency when taking account of the conversion of chemical energy from the primary fuels. The shocking news is that even when boiling water slowly with gas, around half of the energy is wasted! If one is boiling the water quickly then there is no difference in overall efficiency between using gas or electricity – they both waste two thirds of the energy!
- I began by measuring the initial temperature of water from the tap which turned out to be close to 10 Celsius.
- I then weighed the vessel (electric kettle, gas kettle or glass jug ) before and after filling with water.
- I estimated the energy required to raise water from 10 °C to 100 °C using the measured water mass and the heat capacity of water (roughly 4200 J per °C per kg and rather constant in this temperature range).
- For the electric kettle I used its power rating of 2000 W (which I had previously measured and found to be a good estimate.)
- For the gas I used data for the gas burners which indicated they had a rated power of 1 kW, 1.75 kW and 3 kW.
- For the microwave (rated power 800 W) I used a power meter to record the actual consumed power (1240 W)
- I then recorded the time taken for the water to reach 100 °C and compared with the expected time if the device were 100% efficient.
I then factored in the efficiency of the network, assuming that
- For electrical heating: 36% or the original calorific energy of the fuel was delivered to the house.
- For gas: 90% of the original calorific energy of the fuel was delivered to the house, with the remainder being used to power the pumps on the gas pipeline.
Finally on rewriting this in December 2012 I decided to re-check my numbers. I boiled the same kettle on the 1.75 kW burner and got an ‘in-kitchen’ efficiency of 41% compared with 43% that I estimated previously. So I estimate that these numbers have an uncertainty of around 1% or so.