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Key points

• Electric power is the amount of electrical energy converted into other forms of energy in one second.
• Power is measured in watts, W.
• Electrical energy is measured in joules.
• 1 watt = 1 joule per second.
• A joule is too small a unit of energy for household electricity, so the unit used for household electrical energy is the kilowatt-hour, kWh.
• The cost of the electricity used is calculated using this equation: total cost = energy in kWh × cost per unit.
• Higher tier only: Energy in kWh = power rating in kW x time in hours.
• Higher tier only: When using this equation it is important to remember that:
  • the power must be in kW;
  • 1 kW = 1000 W;
  • the time must be in hours.