Energy
Energy is the ability to do work. If a 60 Watt and a 100 Watt electric lamp are compared, the 100 Watt lamp is brighter and therefore does more work.
The joule or watt-second is too small a unit for commercial purposes. In considering domestic loads, the power is measured in kilowatts and the time in hours. A unit called the kilowatt-hour ( kWh ) is used
The kWh is the energy used when 1kW ( 1,000 Watts ) of electric power is consumed for 1 hour or a 100 Watt lamp is on for 10 Hours.
The kWh is referred to as the unit of electricity and it is by this commercial unit that the Supply Authority charges for electrical energy.
Energy Formulae
Example
A 60 Watt lamp is on for 120 minutes. Calculate the amount of electrical energy consumed.
Example
An appliance draws a current of 13 Amps from the supply and has an element of resistance 17R5. How much energy is consumed after a period of 2.5 hours?
Solution
