It took a century to boost their thermal efficiency to around 20 percent in a successive stream of innovations. It took another century again to reach the current steam turbine efficiency of 40 percent. The initial costs of steam engines in the mid-eighteenth century amounted to a phenomenal US$ 12,000 per kW (in 2003$) (Crafts, 2004) in an economy a factor of 130 smaller than today with per capita incomes around US$1,500 (in 2003).