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Electrical energy

The energy a component converts is the power times the time it is on. The electricity bill uses kilowatt hours. One kilowatt hour is the energy when a power of one thousand watts is on for one hour, and it is the same as 3.6 megajoules. If you want to compare with mechanical energy in joules, you must convert the unit. Switch off what you do not use, and both the energy and the bill become smaller.

E=PtE = Ptelectrical energy
1 kWh=3.6 MJ1\ \text{kWh} = 3.6\ \text{MJ}conversion

Symbols

EEenergyJ or kWh
PPpowerW or kW
tttimes or h

Example

A heater of 2 kW is on for 3 hours.

E=2⋅3=6E = 2 \cdot 3 = 6 kWh =6⋅3.6=21.6= 6 \cdot 3.6 = 21.6 MJ.

Use kilowatts and hours together, or watts and seconds together. Do not mix the two.
Practise electricity for free →

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Part of Physics 1: Electricity.