The cost of a cuppa

At some point in the near future I was planning to do an article about the cost of tea, taking into account the cost of the tea bag, milk and also the energy required to heat the water. However I put aside my original plan for today when I read this article: http://uk.finance.yahoo.com/blogs/british-gas/microwave-tea-114714870.html

Not that it mentions it within the article, but the title claims that we should microwave our tea, presumably to cut down on energy costs. This has prompted me to spend today’s blog energy investigating.

During day 7’s cup of tea, the scientist in me took over and I started to make a plan. I have 2 questions to answer:

1. How much does it cost to make a cup of tea (white, no sugar with a kettle)

2. Which uses more energy: boiling water in a kettle or in a microwave?

1. How much does it cost to make a cup of tea (white, no sugar with a kettle)

The cost of the tea bag depends on the variety bought. So going on the basis of a box of 80 Sainsbury’s red label tea being £1.39, 1 tea bag costs 1.7p (mysupermarket.co.uk correct as of 21/2/12). The cost of the milk would also vary. For me, I’d put about 40ml of milk in a mug of tea. If I got my milk from Sainsbury’s again and got 4 pints of normal milk (green milk) that would be £1.18, and the nice website people tell me it is 5.5p per 100 ml. So 40ml of milk is 2.2p.

My 2000W kettle containing a mug’s worth of hot water took 64 seconds to boil (well, the water came out at 85 degrees C). To calculate the cost we need to use this information to calculate the energy used in kilowatt hours (kWh) by multiplying the power in kW by the time in hours. According to the article, the average cost of a kWh is 14.39p.

So 2 kW x 0.0178h = 0.0356 kWh. Then 0.0356×14.39p = 0.51p worth of electricity for a cup of tea. So my month of tea (lets say a long month of 31 days) will increase my electricity bill by 15p, but only if I only put just enough water for my cup in the kettle.

So the cost of a cuppa: 0.51+2.2+1.7 = 4.41p. This includes the teabag, milk and heating the water. It does not include the cost of the water, the cost of going to collect the goods from a shop (not the mention the other things the shop successfully tempts you to buy that you never intended to get, if you’re anything like me), the cost of running a fridge to keep your milk at a suitable temperature, the cost of any sugar, the cost of the cup, the cost of washing the cup up and anything else I’ve forgotten that wouldn’t be especially easy to include.

2. Which uses more energy: boiling water in a kettle or in a microwave?

To investigate this, I can calculate the energy used in the kettle and compare to that of a microwave. Instead of calculting the energy in kWh, which while easier to convert into pence, leaves us with many decimal places, I’m going to calculate the amount in kilo joules (a name dear to my heart, although it’s spelt wrong).

So power in kW x time in seconds = energy in kJ. So 2 x 64 = 128kJ

My microwave has a power of 0.8kW. So for my microwave to be as good if not better than my kettle, it would need to use 128kJ or less to heat the water to the same temperature (or higher). So ?seconds = 128kJ/0.8kW = 160 seconds or 2 min 40 sec.

So I placed the same volume of water in the mug in the microwave and heated it for 2 min 40, and after that time the water was about 82 degrees C. This makes the microwave not as good as the kettle.

I repeated the experiment again later (I started writing this around 6pm but then had to go out, so I have since returned). This time the kettle took 67 seconds to boil, and the mug of water in the microwave was 88 degrees C after 2 min 40.

My first experiment showed the kettle to use slightly less energy to heat the water than the microwave. The second showed the kettle to use slightly more energy to heat the water than the microwave. This indicates 2 things. First of all, I should carry out further repeats to really determine what is happening.

Secondly, there is not a great deal of difference between the kettle and the microwave. Lets take the second experiment. The cost of running my microwave for 2 min 40 is 0.5116p. The cost of running my kettle for 67 seconds is 0.5356p. So the difference is 0.024p. If you were to drink 3 cups a day for a year, this would amount to a saving of 26.38p per year.

That’d buy you a pack of paracetamol and a 10p sweet.