Few people know how much the water from their cold tap costs, and even fewer realise how much the water from their hot tap costs. The cost of cold water in the UK is around a quarter of a penny per liter, so that's our starting point. How much more does it cost to heat the water?
We'll heat the water to 40 degrees Celsius. If the water comes from underground piping at, say, 10°C, we must raise the temperature of the water by 30 degrees. It takes 4.187 kilojoules (kJ) of energy to heat a liter of water by one degree, so it would need 125 kJ to heat a liter of water from 10 to 40 degrees.
The price of energy
In the UK, I pay around 20 pence per kilowatt-hour (3.6 Megajoules) of on-demand electricity.
If I had an off-peak supply, I would pay around 5 pence per kilowatt-hour, but there are heat losses from tanks that store off-peak hot water. Those losses are not directly proportional to the amount of hot water used, so I'm going to include
a somewhat arbitrary extra penny per kilowatt-hour which brings the cost of off-peak electricity to 6 pence per kW-hr.
I pay around 8 pence per kilowatt-hour for gas, but a gas heater is only around 90% efficient so I will use a figure of 9 pence per useful kilowatt-hour.
How much does a basinful of hot water cost?
If the basin holds 20 liters, the cold water will cost 5 pence.
Heating that water with on-demand electricity will add 14p. Heating it with on-demand gas will add 6 pence, and heating it with off-peak electricity will add 4 pence.
What about the hot water in a ten-minute shower?
Unless you have a “water-saving” shower that dispenses a misty drizzle, a ten-minute shower will use around a hundred liters of water, which will cost 25p.
Electric heating will add 69p (or 21p if from off-peak electricity), and gas heating will add 31p. The total cost of the shower will be between 46p and 94p.
What about the hot water in a cup of coffee?
Let's assume 200ml of water, heated from 10 to 100ºC. The water itself costs 0.05 pence (one-twentieth of a penny). Electric heating would add 0.42 pence and gas heating would add 0.19 pence. Either way, the total cost of the hot water in a cup of coffee will be less than half a penny.
At the time of writing, one British pound (of 100 pence) equals $1.50 US.
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