If the electricity supplier charges \$C\$ (currency per kWh), the cost \$K\$ of using an appliance is
\$K = E_{\text{kWh}} \times C\$
where \$E_{\text{kWh}}\$ is the energy consumed expressed in kilowatt‑hours.
Step‑by‑step calculation
Find the power rating of the appliance (usually given in watts, W).
Convert the power to kilowatts: \$P{\text{kW}} = \dfrac{P{\text{W}}}{1000}\$.
Determine how long the appliance is used (in hours), \$t_{\text{h}}\$.
Calculate the energy used: \$E{\text{kWh}} = P{\text{kW}} \times t_{\text{h}}\$.
Multiply by the unit price of electricity to obtain the cost.
Example
A 1500 W electric heater is switched on for 3 hours each day. The electricity price is £0.20 per kWh.
Power in kilowatts: \$P_{\text{kW}} = 1500/1000 = 1.5\;\text{kW}\$.
Energy per day: \$E_{\text{kWh}} = 1.5\;\text{kW} \times 3\;\text{h} = 4.5\;\text{kWh}\$.
Cost per day: \$K = 4.5\;\text{kWh} \times £0.20/\text{kWh} = £0.90\$.
Cost per month (30 days): \$£0.90 \times 30 = £27.00\$.
Typical household appliances
Appliance
Power rating (W)
Daily use (h)
Energy per day (kWh)
Cost per day (at £0.20/kWh)
LED lamp (5 W)
5
5
\$\dfrac{5}{1000}\times5 = 0.025\$
£0.005
Refrigerator (150 W)
150
24
\$\dfrac{150}{1000}\times24 = 3.6\$
£0.72
Electric kettle (2000 W)
2000
0.5
\$\dfrac{2000}{1000}\times0.5 = 1.0\$
£0.20
Television (100 W)
100
4
\$\dfrac{100}{1000}\times4 = 0.40\$
£0.08
Washing machine (800 W)
800
1
\$\dfrac{800}{1000}\times1 = 0.80\$
£0.16
Suggested diagram: A simple schematic showing a power rating label on an appliance, a clock indicating usage time, and a calculator displaying the cost calculation.
Key points to remember
1 kWh = 3.6 MJ (megajoules).
Always convert watts to kilowatts before using the kWh formula.
The cost of running an appliance depends on both its power rating and the duration of use.
Reducing the time an appliance is on, or using lower‑power alternatives, directly lowers the electricity bill.