Section 1 of 9What power means
Power is the rate at which energy is transferred. An appliance with a power rating of 1 kW transfers 1000 joules of energy every second. The SI unit is the watt (W), where 1 W = 1 J s−1. Larger values are given in kilowatts (kW) or megawatts (MW).
The rating printed on an appliance, such as "230 V, 2 kW", tells you the voltage it is designed for and the power it transfers when connected to that voltage.
Section 2 of 9The electrical power equations
When a current flows through a component, the power transferred depends on how much current flows and the potential difference across the component:
P = I V
Combining this with V = I R gives two further forms that are useful when the resistance is known:
Notice that P = I2R shows that doubling the current in a fixed resistor makes the power four times larger. This is why thin wires carrying large currents become hot very quickly.
Section 3 of 9Why we need fuses
If a fault allows too much current to flow, the wires heat up and can start a fire.
The fuse is always connected in the live wire, so that when it blows the appliance is cut off from the high voltage side of the supply. If it were in the neutral wire, the appliance would stay live even though no current flowed.
Section 4 of 9Choosing the correct fuse
Domestic fuses are made in standard ratings, commonly 3 A, 5 A and 13 A. The right fuse is the one with a rating just above the normal working current of the appliance. Too low and the fuse blows during normal use; too high and it will not protect the wire from overheating.
To find the normal current, rearrange P = I V to give I = P / V and use the mains voltage (230 V in the UK and many other countries). Then pick the next standard fuse rating above the answer.
Section 5 of 9Fuses, circuit breakers and links
A circuit breaker does the same job as a fuse but is an electromagnetic switch that trips when the current is too high.
Section 6 of 9Power in everyday appliances
Comparing power ratings tells you how quickly appliances transfer energy.
- and an electric shower at 8 to 10 kW.
For example, a 3 kW heater draws I = 3000 / 230 ≈ 13 A, which is right at the limit of a 13 A fuse. Plugging several such appliances into one extension lead can easily overload it, since the currents add together in the same cable.
The power equations also apply to every component in a circuit, not just mains appliances.
When you tackle a power question, first list what you know (P, V, I or R), then choose the equation that contains exactly those quantities plus the one you want.
Section 7 of 9Worked examples
A 920 W kettle works from a 230 V supply. Find the current and choose a fuse from 3 A, 5 A and 13 A.
I = P / V = 920 / 230 = 4.0 A.
The fuse must be just above 4.0 A, so the 5 A fuse is chosen. A 3 A fuse would blow in normal use.
A heating element has resistance 46 Ω and a current of 5.0 A flows through it. Calculate the power.
P = I2 R = 5.02 × 46 = 25 × 46 = 1150 W (about 1.2 kW).
A 12 V lamp transfers 36 W. Calculate the current and the resistance of the lamp when it is lit.
I = P / V = 36 / 12 = 3.0 A.
R = V / I = 12 / 3.0 = 4.0 Ω.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Power | Rate of energy transfer; measured in watts |
| Watt | One joule per second |
| Fuse | Thin wire that melts when current is above its rating |
| Rating | The current at which a fuse is designed to melt |
| Circuit breaker | Resettable switch that trips when current is too high |
| Live wire | Wire carrying the alternating high voltage to the appliance |