# 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.

230 V × 13 A = 3000 W