This chapter explores how much power we spend controlling the temper-
ature of our surroundings – at home and at work – and on warming or
cooling our food, drink, laundry, and dirty dishes.
The biggest use of hot water in a house might be baths, showers, dish-
washing, or clothes-washing – it depends on your lifestyle. Let’s estimate
first the energy used by taking a hot bath.
The volume of bath-water is 50 cm × 15 cm × 150 cm ≈ 110 litre. Say
the temperature of the bath is 50 °C (120 F) and the water coming into the
house is at 10 °C. The heat capacity of water, which measures how much
energy is required to heat it up, is 4200 J per litre per °C. So the energy
required to heat up the water by 40 °C is
4200 J/litre/°C × 110 litre × 40 °C ≈ 18 MJ ≈ 5 kWh.
So taking a bath uses about 5 kWh. For comparison, taking a shower
(30 litres) uses about 1.4 kWh.
Britain, being a civilized country, has a 230 volt domestic electricity supply.
With this supply, we can use an electric kettle to boil several litres of water
in a couple of minutes. Such kettles have a power of 3 kW. Why 3 kW?
Because this is the biggest power that a 230 volt outlet can deliver with-
out the current exceeding the maximum permitted, 13 amps. In countries
where the voltage is 110 volts, it takes twice as long to make a pot of tea.
If a household has the kettle on for 20 minutes per day, that’s an average
power consumption of 1 kWh per day. (I’ll work out the next few items
“per household,” with 2 people per household.)
One small ring on an electric cooker has the same power as a toaster:
1 kW. The higher-power hot plates deliver 2.3 kW. If you use two rings
of the cooker on full power for half an hour per day, that corresponds to
1.6 kWh per day.
A microwave oven usually has its cooking power marked on the front:
mine says 900 W, but it actually consumes about 1.4 kW. If you use the
microwave for 20 minutes per day, that’s 0.5 kWh per day.
A regular oven guzzles more: about 3 kW when on full. If you use the
oven for one hour per day, and the oven’s on full power for half of that
time, that’s 1.5 kWh per day.