What is domestic single phase electricity bill fundamental? – what is a Unit?

Basically we pay electricity bill as per numbers of units multiple with a specific unit charge. Now we try to understand what a unit is and how to calculate it with our electrical equipments used power?

Every electrical equipment specified with a rated power, for example 10watt LED bulb, 100watt fan, etc.

Energy = Power x Time

 The unit of energy is the joule, the unit of power is the watt, and the unit of time is the second.

When we know what is the power of our appliance and the used time, we can calculate numbers of joules of used electrical energy. For a example we used one 200watt bulb and 1000watt fan for 2 hours.

Energy (w) = Power x Time

Energy = (200 + 1000 ) * 7200

 = 8640000 joules  =  8640 kJ

It’s means one watt is equal to one joule in one second.

As we seen that joule is a very small unit and if we calculate as per joule is will be a long value, so the electrical energy supplied to customer in UNIT. A unit is equal to one kilowatt hour (kWh).

As per previous example value, we are using one 200watt bulb and 1000watt fan for 2 hours.

Power in kW = (200 + 1000) / 1000

                            = 1.2 kW

Energy in kWh  = kW x Time in hours

                                =  1.2 * 2

                                =  2.4 kWh

                                Or 2.4 units in 2 hours