By 1969, American domestic output of oil could not keep pace with increasing demand. In 1925, oil had accounted for one-fifth of American energy use; by the time 1941 started, one-third of America's energy needs was met by oil. Oil started to replace coal as a preferred fuel source. It was used to heat homes and generate electricity, and it was the only fuel that could be used for air transport. In 1920, American oilfields accounted for nearly two-thirds of global oil production. In 1945, US production had increased to just over two-thirds. The US had been able to meet its own energy needs independently in the decade between 1945 and 1955, but was importing 350 million barrels per year by the late 1950s, mostly from Venezuela and Canada. In 1973, US production had declined to 16.5% of global output.