How much does it cost to make a cup of tea?

Discuss energy-efficiency and energy wastage with students, and demonstrate how simple behavioural changes can have an impact on an individual’s energy consumption.

LESSON OBJECTIVES

  • Understand how to measure the electricity used by an electrical item
  • Calculate the cost of the electricity used to boil a kettle
  • Work out the difference in cost and energy used when boiling a full, two-thirds full and one-cup full kettle.
  • Think about the implications of this and how individual behaviour affects energy consumption

Time required: 30 minutes

Report these findings in simple graph form

Resources & preparation required

Kettle, water, mug, stopwatch, hand held monitoring device.

Introduction

Ask students to think about the popularity of tea/coffee in the UK and how many cups they think the nation drinks each day. According to the UK Tea Council, around 165 million cups of tea are drunk daily (and only 70 million cups of coffee). Their website has a counter showing the number of cups of tea consumed in the UK so far each day: www.tea.co.uk.

Electric_Kettle_PNG_Clipart-637This exercise will show how much energy is used by boiling the kettle to make one of these cups of tea or coffee, and how much that costs.

Ensure that the kettle is full and then plug it in to the energy monitor. Explain to students that the socket or strip contains a smart meter, which will measure the amount of electricity that is passing through it – i.e. the amount used by the kettle.

Show them the graph of energy usage for the kettle. It should show that no power is currently being consumed.

Switch on the kettle, start the stopwatch, and watch the graph change. Record a figure, in Watts, of how much power is used (it will fluctuate a little but you should be able to record a rough average), along with the time taken to boil the kettle. Explain to students that this is not the total energy used but the rate of energy usage.

Electrical power is measured in Watt-hours (Wh) or Kilowatt-hours (kWh).

To measure the energy used, you need the formula:

Energy = Power (in W or kW) x Time (in hours)

E.g. if the kettle uses 2180W and takes 2 minutes to boil, its energy usage is:

2180 x (2 ÷ 60) = 72.6Wh or 0.0726 kWh.

Repeat the exercise, but with the kettle only two-thirds full of water, and then with the kettle just holding enough water for one mug of tea.

energy-graphCalculate the total energy used for each scenario and ask students to draw a simple bar chart to show the comparison. How much energy is wasted if only one mug of water is wanted but a full kettle is boiled?

For each case, work out the costs of the energy used (basing the calculation either on your school’s actual electricity tariff, or using the average cost of 10p per kWh) and add them to the bar chart. So for the example above, the cost would be 10p x

0.0726 = 0.007p.

Question: Robin drinks four cups of tea every day and always fills his electric kettle completely before boiling it. Assume that his kettle is exactly the same as the one you have used for the exercise above. How much money could Robin save each week, and then in a whole year, by only boiling enough water for one cup of tea?

Discussion / summing up

Is the amount of energy used by boiling unnecessary water a significant amount?

Ask students to consider how often a kettle is boiled – for cooking as well as for hot drinks. Could each person really save money, and help the environment, by only boiling as much water as they need?

How could students tell others (e.g. teachers, parents) about their findings to encourage them to be more environmentally responsible on a daily basis?

Extension / homework suggestion

  1. On average, people in the UK drink 165 million cups of tea every single day. If a kettle like the one you have used for this exercise is used for each of those cups, how much electricity is used every day? Come up with two answers – one for if everyone fills the kettle completely before boiling and one for if they just boil enough water for one cup. What would this cost and how much is saved by only boiling the water needed for one cup?
  2. Design a poster explaining the benefits of thinking before you put on the kettle, and only using as much water as you really need.

Risk assessment

The supervising teacher should carry out all risk assessments relating to this activity, and ensure that they comply with the requirements of the institution in which the lesson is conducted.


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