An obsolete unit of work or energy equal to the work done at the rate of 1 horsepower for 1 year. Based on the Julian year of 365.25 days, the horsepower year is approximately 6536.8 kilowatt hours, 23.537 gigajoules, or 22.309 million Btu
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}