Skip to content
Grade 10 Intermediate About 20 min Version 1 Awaiting review Premium

Electrical power and the electricity bill

Power is how fast an appliance uses electrical energy, measured in watts (W); 1000 W = 1 kW. Its power is printed on its label. The energy used depends on the power and the time it runs: energy (kWh) = power (kW) × time (hours). One kilowatt-hour is one "unit" on the electricity bill. The current an appliance draws from the 230 V mains is I = P ÷ V. Powerful heating appliances like kettles and irons use a lot of energy even in a short time, while LED bulbs use very little.

What you will learn

  • Calculate energy used in kWh from power and time.
  • Calculate the monthly cost of running an appliance.
  • Compare the energy use of different appliances and suggest ways to save energy.

Before you start

  • Know that power is measured in watts.

Materials

  • Labels showing the power of household appliances

Equipment

  • Electricity meter (or a plug-in energy meter)

Procedure

  1. Run the LED bulb for 4 hours a day and record the monthly units and cost.
  2. Replace it with the old filament bulb for 4 hours a day and compare.
  3. Run the electric kettle for half an hour a day and record.

How this simulation is simplified

  • Mains voltage is 230 V and a month has 30 days.
  • A simple flat rate of Rs 10 per unit is used; real NEA tariffs depend on how many units you use.