Electricity: Power

Electric power is the rate at which electrical energy is converted to another form of energy. It is measured in watts (W) and is given by the product of voltage(V) and current(I): P = V x I.
High power devices, such as an electric kettle or a water heater, convert a lot of electrical energy into another form of energy (heat in this case) per second. Hence, they have a high wattage rating.
The power of a device can also be determined using two other formulas that are derived from Ohm’s law (V = I x R). Here R is resistance. These are: P = I² x R and P = V² / R.
Kilowatt-hours (kWh) is a unit used to measure energy. One kWh is the energy consumed by a 1 kW (1000 watts) device running for one hour. This is typically used to measure the amount of energy households consume.
When an electrical device is used, it consumes energy. The energy E (in Joules) consumed by a device with power P (in watts) in a time t (in seconds) is given by E = P x t.
The cost of running electrical appliances can be calculated by finding the number of kilowatt-hours used, and then multiplying this by the cost per kilowatt-hour.
Two devices that have the same power rating will not necessarily consume the same amount of electricity over a certain period. The actual energy consumed also depends on how long the devices are used for.
Fuses are safety devices in electrical circuits. They contain a thin wire that heats up and melts if too much current passes through. This breaks the circuit and prevents potential hazards.
The circuit breaker is another device that serves a similar function to the fuse. It breaks the circuit if there is a sudden large increase in current (such as in a short circuit). The main advantage of a circuit breaker over a fuse is that it can be easily reset, while a blown fuse has to be replaced.
The Earth wire in a plug is connected to the case of the appliance. If live wire touches the case, the current flows to earth, this large current melts the fuse or trips the circuit breaker, disconnecting the appliance. This prevents the case from becoming live and posing a shock hazard.
The National Grid uses transformers to step up the voltage for transmission and step down for use in our homes. This is because a high transmission voltage reduces power loss during long-distance transportation.