In 1800, coal combustion in stoves and boilers to produce heat and hot water was no more than 25–30 percent efficient, and only 2 percent of coal consumed by steam engines was converted into useful work, resulting in an overall conversion efficiency of no higher than 15 percent. A century later, better stoves, boilers, and engines raised the overall efficiency to nearly 20 percent, and by the year 2000 the mean conversion rate was about 50 percent. Consequently, the 20th century saw a nearly 40-fold gain in useful energy; since 1800 the gain was about 3,500-fold.
