Lookup vs. Memory

Somehow I misremembered one horsepower to be 550 watts which is wrong. But I feel better knowing others differ as well. I'm going with google: one horsepower = 745.699872 watts. I can remember 746. Here's what Wikipedia has to say:

Units called "horsepower" have differing definitions:

• The mechanical horsepower, also known as imperial horsepower, of exactly 550 foot-pounds per second is approximately equivalent to 745.7 watts.
• The metric horsepower of 75 kgf-m per second is approximately equivalent to 735.499 watts.
• The boiler horsepower is used for rating steam boilers and is equivalent to 34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, or 9,809.5 watts.
• One horsepower for rating electric motors is equal to 746 watts.
• Continental European electric motors used to have dual ratings, using a conversion rate of 0.735 kW for 1 hp
• British Royal Automobile Club (RAC) horsepower is one of the tax horsepower systems adopted around Europe which make an estimate based on several engine dimensions.

This is interesting: a healthy human can produce about 1.2 hp briefly and sustain about 0.1 hp indefinitely; trained athletes can manage up to about 2.5 hp briefly and 0.3 hp for a period of several hours.