Energy used by an appliance
Every electrical appliance has a power rating printed on it, telling you how much electrical energy it uses each second, in watts (W). A larger rating means the appliance uses energy faster. The energy used is found from E = P × t, where P is the power and t is the time it is switched on.
Key idea
Energy (kWh) = Power (kW) × Time (h). One kilowatt-hour (kWh) equals one unit of electricity on the bill.
The kilowatt-hour
Household energy is measured in kilowatt-hours (kWh), not joules, because joules would give huge numbers. First change the power from watts to kilowatts by dividing by 1000, then multiply by the time in hours. A 1000 W (1 kW) heater used for 2 hours uses 1 × 2 = 2 kWh, or 2 units.
Finding the cost
The electricity company charges a fixed price for each unit, called the tariff. The cost of using an appliance is simply the number of units multiplied by the tariff.
Example
A 2000 W (2 kW) iron is used for 3 hours. Energy = 2 × 3 = 6 kWh. If the tariff is 40 sen per unit, cost = 6 × 40 = 240 sen = RM 2.40.
Remember
- Change watts to kilowatts by dividing by 1000.
- 1 kWh = 1 unit of electricity.
- Cost = units × tariff.