A Watched Pan…

Click on Image for larger version.  A vision of domestic bliss in the de Podesta household. Apparatus in use for measuring the rate of heating of 1 litre of water on an induction hob.

In the beginning…

Friends, my very first blog article (written back on 1st January 2008 and re-posted in 2012) was about whether it is better to boil water with an electric kettle or a gas kettle on a gas hob.

Back then, my focus was simply on energy efficiency rather than carbon dioxide emissions. I had wanted to know how much of the primary energy of methane ended up heating the water. I did this by simply timing how long it took to boil 1 litre of water by various methods.

Prior to doing the experiments I had imagined that heating water with gas was more efficient because the fuel was used directly to heat the water. In contrast, even the best gas-fired power stations are only ~50% efficient.

What I learned back then was that gas cookers are terrible at heating kettles & pans! They were so much worse than I had imagined that I later spent many hours with different size pans, burners, and amounts of water just so I could believe my results!

Typically gas burners only transferred between 36% and 56% of the energy of combustion to the water – the exact fraction depending on the size and power of the burner. Heating things faster with a bigger burner was less efficient. Using a small flame and a very large pan, I could achieve an efficiency of 83%, but of course the water heated only very slowly.

This inefficiency was roughly equivalent to or worse than the inefficiency of the power station generating electricity, and so I concluded that electric kettles and gas kettles were similarly inefficient in their use of the primary energy of the gas. But that using electric kettles allowed one to use the correct amount of water more easily, and so avoided heating water that wasn’t used.

14 years later…

After a recent conversation on Twitter (@Protons4B) I thought I would look at this issue again.

Why? Well two things have changed in the last 14 years.

• Firstly, electricity generation now incorporates dramatically more renewable sources than in 2008 and so using electricity involves ever decreasing amounts of gas-fired generation.
• Secondly, I am now concerned about emissions of carbon dioxide resulting from lifestyle choices.

Also being a retired person, I now have a bit more time on my hands and access to fancy instruments such as thermometers.

The way I did the experiments is described at the end of the article, but here are the results.

Results#1: Efficiency

The chart below shows estimates for the efficiency with which the electrical energy or the calorific content of the gas is turned into heat in one litre of water. My guess is these figures all have an uncertainty of around ±5%.

• The kettle was close to 100% efficient:
• The induction hob was approximately 86% efficient
• The Microwave oven was approximately 65% efficient

In contrast, heating the water in a pan (with a lid) on a gas hob was only round 38% or 39% efficient.

Click on Image for larger version. Chart showing the efficiency of 5 methods of heating 1 litre of water. 100% efficiency means that all the energy input used resulted in a temperature rise. The two gas results were for heating pans with two different diameters (19 cm and 17 cm).

It was particularly striking that the water heated on the gas burner (~1833 W) took 80% longer to boil than on the Induction hob (~1440 W) despite the heating power being ~20% less on the induction hob.

Click on Image for larger version. Chart showing the rate of heating for each of the 5 methods of heating 1 litre of water. Notice that the water heated on the gas burner (~1833 W) took 80% longer to boil than on the Induction hob (~1440 W) despite the heating power being ~20% less on the induction hob. Notice that up until 40 °C, the microwave oven heats water as fast as the gas hob, despite using half the power!

Results#2: Carbon Dioxide Emissions 

Based on the average carbon intensity of electricity in 2021 (235 g CO2/kWh), boiling a litre of water by any electrical means results in substantially less CO2 emissions than using a pan (with a lid) on a gas burner.

I performed these experiments on 17th January 2021 between 4 p.m. and 7 p.m. when the carbon intensity of electricity was well above averages: ~330 g CO2/kWh. In this case, boiling a litre of water in a kettle or induction hob still gave the lowest emissions, but heating water in a microwave oven resulted in similar emissions to those arising from using a pan (with a lid) on a gas burner.

Click on Image for larger version. Charts showing the amount of carbon dioxide released by heating 1 litre of water from 10 °C to 100 °C using either electrical methods or gas. The gas heating is assumed to have a carbon intensity of 200 gCO2/kWh. The left-hand chart is based on the carbon intensity of 330 gCO2/kWh of electricity which was appropriate at the time the experiments were performed. The right-hand chart is based on the carbon intensity of 235 gCO2/kWh of electricity which was the average value for 2021. Electrical methods of heating result in lower CO2 emissions in almost all circumstances.

Results#3: Cost 

Currently I am paying 3.83 p/kWh for gas and 16.26 p/kWh for electricity i.e. electricity is around four times more expensive than gas.

These prices are likely to rise substantially in the coming months, but it is not clear whether this ratio will change much.

So sadly, despite gas being the slowest way to heat water and the way which releases the most climate damaging gases, it is still the cheapest way to heat water. It’s about 40% cheaper than using an electric kettle.

Conclusion 

For the sake of the climate, use an electric kettle if you can.

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That was the end of the article and there is no need to read anymore unless you want to know how I made these measurements.

Method 

Estimating the power delivered to the water + vessel

• For electrical measurements I paused the heating typically every 30 seconds, and read the plug-in electrical power meter. This registered electrical energy consumed in kWh to 3 decimal places.
  • I fitted a straight line to the energy versus time graph to estimate power.
• For gas measurements I read the gas meter before and after each experiment. This reads in m^3 to 3 decimal places and I converted this volume reading to kWh by multiplying 11.19 kWh/m^3.
  • The gas used only amount to 0.025 m^3 so uncertainty is at least 4% from the digitisation.
  • I divided by the time – typically 550 seconds – to estimate the power.

Mass of water

• I placed the heating vessel (kettle, pan, jug) on the balanced and tired (zeroed) the reading.
• I then added water until the vessel read within 1 g of 1000 g. Uncertainty is probably around 1% or 10 g.

Heating rate with 100% energy conversion

• Based on the power consumed, I estimated the ideal heating rate if 100% of the supplied power caused temperature rises in the water by using the equations.

• I assumed the average specific heat capacity of water of the range from 10 °C to 100 °C was 4187 J/ (kg °C)

Measuring the temperature.

• For electrical measurements I paused the heating typically every 30 seconds, stirred the liquid with a coffee-stirrer for 2 or three seconds, and then took the temperature using a thermocouple probe..
• For gas measurements it wasn’t possible to the pause the heating because of the way I was measuring the power. So about 10 seconds before the reading was due I slipped the coffee stirrer under the lid to mix the water.

Estimating the rate of temperature rise.

• For all measurements I fitted a straight line to the temperature versus time data, using only data points below approximately 80 °C to avoid the effects of increased temperature losses near to the boiling point.

Mass of the ‘addenda’.

• The applied power heated not only the water but also its container.
• The heat capacity of the 19 cm stainless steel pan (572 g) was roughly 6% of the heat capacity of the water.
• I chose not to take account this heat capacity because there was no way to heat water with a container. So the container is a somewhat confounding factor, but allows more meaningful comparison of the results.

Efficiency of boiling

• I estimated efficiency by comparing the actual warming rate with the ideal warming rate.
• I then calculated the energy required to heat 1 kg of water from 10 °C to 100 °C, and multiplied this by the efficiency.
• In this way the result is relevant even if all the measurements did not start and stop at the same temperatures.

Oddities

• I heated the water in the microwave in a plastic jug which did not have a tight fitting lid. I am not sure if this had an effect.
• I did notice that the entire microwave oven was warm to hot at the end of the heating sequence.