Question

why are we not using joule as commercial unit ?

Answer 1

for electric utilities, the kilowatt-hour (kWh) is the most common billing unit for energy delivered to consumers. A kilowatt-hour is a unit of energy equivalent to one kilowatt of power expended for one hour of time. It is preferred ove joule unit because normally the power of the bulb and other equipments are given in W or kW. So it is easy to understant how much electricity we have used.
To calculate kWh, you take the wattage of the device, multiply that by the number of hours that you predict it's used, and divide by 1,000. For example, says Mathot, if you use a 100-watt incandescent bulb for 10 hours, then you have consumed 1 kWh of electricity.