James Watt

The concept of Power was developed by James Watt for the purpose of advertising steam engines for pumping water out of tin mines in Cornwall in England.

 Horsepower

In modern terms Watt's horsepower is 746 Watts - a little more than the average horse can produce over a full working day.

 Manpower

A manpower unit could be defined. It would be about 100 Watts, which, like the horsepower, is on the optimistic side.