Calculating the Cost of Electricity

Electricity bills are calculated using the kilowatt-hour (kWh), not the joule.

The kilowatt-hour

$E(\text{kWh}) = P(\text{kW}) \times t(\text{h})$

1 kWh is the energy transferred by a 1 kW device operating for 1 hour.

$1 \text{ kWh} = 1000 \text{ W} \times 3600 \text{ s} = 3{,}600{,}000 \text{ J} = 3.6 \text{ MJ}$

Calculating energy used

Example: A 2.5 kW oven runs for 1.5 hours.

$E = 2.5 \times 1.5 = 3.75 \text{ kWh}$

Calculating cost

$\text{cost} = E(\text{kWh}) \times \text{price per kWh}$

If electricity costs RM 0.40 per kWh:

$\text{cost} = 3.75 \times 0.40 = \text{RM } 1.50$

Comparing appliance running costs

Appliance	Power (kW)	Time used per day (h)	Energy per day (kWh)
LED bulb	0.01	8	0.08
Laptop	0.06	6	0.36
Kettle	2.0	0.25	0.50
Air conditioning	1.5	10	15.0

The air conditioning uses far more energy despite having a lower power rating than the kettle, because it runs for much longer.

Common errors and how to correct them

• Using watts instead of kilowatts (divide by 1000 first).
• Using seconds instead of hours.
• Forgetting to multiply by the price per unit.

How to apply this in an exam

Convert power to kW and time to hours. Multiply for energy in kWh. Multiply by the cost per unit. Show each step clearly.