If you need to replace a broken domestic light bulb you will need to take note of the two sets of figures printed on the glass. One of the figures is likely to be 240 V. This number confirms that for the bulb to work correctly it should be connected to the UK mains supply. The second set of figures could be 10 W or 60 W or 100 W, or similar. But what do these figures mean? If you bought one of each of these bulbs and connected them to the same supply the difference between them would be obvious. The 10 W (ten watt) bulb would be quite dim, the 60 W bulb would be brighter, and the 100 W bulb would glow even brighter.
Bulbs are energy converters. They convert electrical energy into heat and light energy. The numbers on the bulbs tell us how quickly they do this. The 100 W bulb is very bright as it is converting 100 J (joules) of electrical energy into 100 J of heat and light energy every second it is turned on. The 60 W bulb is less bright as it is converting 60 J of electrical energy into 60 J of heat and light energy every second. The 10 W bulb is dim as it is converting just 10 J of electrical energy into 10 J of heat and light energy every second.
The rate at which a device converts energy is called the power or power rating of the device. For example, the brightest bulb above has a power rating of 100 W because it converts energy at the rate of 100 J every second. The dullest bulb has a power rating of 10 W because it converts energy at the rate of 10 J every second.
power and energy It follows from the above that we can work out how much energy a device has converted by using the equation:
energy converted = power in watts × time in seconds
or E = P × t
The energy calculated is measured in joules.
example Calculate how much energy is used when a 600 W TV set is turned on for 30 minutes, using E = P × t.
E = 600 × (30 × 60)
E = 1,080,000 J or 1,080 kJ
The energy used in joules when a TV set is turned on for just 30 minutes is quite a large number. If we measure the total amount of electricity we use in the home over three months in joules the number would be incredibly large and clumsy. (A little bit like asking you how far it is to the town centre from where you live but asking for the distance in centimetres!)
To get around this problem we usually measure domestic electrical energy in much bigger units, called the kilowatt-hour or unit.
Imagine a 1 kW fire turned on for one hour – it will use 1 kWh (kilowatt-hour) or 1 unit of energy. If it is turned on for two hours it will use 2 kWh or 2 units of energy. If a 2 kW fire is turned on for three hours it will use 6 kWh or six units of energy, and so on.
Therefore we can calculate the energy used in kWh or units by any device, using the equation:
number of units used = power of device in kW × time in hours
To work out the total cost of the electricity we have used, we now simply multiply the number of units used by the cost of one unit.
example Calculate the cost of having a 3 kW electric fire turned on for one full day if the cost of one unit is 9 p, using E = P × t.
E = 3 × 24
E = 72 units
total cost = number of units × cost per unit
total cost = 72 × 9
total cost = 648 p or £6.48