Watts are joules per second. 1 kWh is 3,600,000 Joules but it is more convenient to use some unit of watt because the power rating of the things you're attachi

Watts are joules per second. 1 kWh is 3,600,000 Joules but it is more convenient to use some unit of watt because the power rating of the things you're attaching it to are often rated in Watts also. Besides it is just the way things evolved. Like batteries being rated in mAh.