7   Heating and cooling

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.

Domestic water heating

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.

Kettles and cookers

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.

Figure 7.1. A flock of new houses.
Figure 7.2. The water in a bath.
230 V × 13 A = 3000 W
Figure 7.3. Power consumption by a heating and cooling device.