MEPX
Chapter 5 of 10All chapters
  1. 1Charge and current
  2. 2Voltage
  3. 3Resistance and Ohm law
  4. 4Series and parallel
  5. 5Power and energy
  6. 6Components
  7. 7AC and DC
  8. 8Electrical safety
  9. 9Reading a circuit diagram
  10. 10Everyday electricity

Chapter 5 of 10

Power and energy

What the bill measures.

Power

Power is voltage times current, in watts, and can also be written as current squared times resistance. That squared term is why halving current cuts losses fourfold.

  • A kilowatt hour is a thousand watts for an hour, the unit on an electricity bill.
  • A 2 kW heater run for 3 hours uses 6 kilowatt hours.

Efficiency

Every device turns some input into unwanted heat. Comparing rated power against useful output is what efficiency ratings describe.