The results are also broken down by source.

The line graph shows growth in the consumption of renewable energy during the period 1949-2008 in the USA. The results are also broken down by source. The first thing to note is that renewable energy use more than doubled over the period, with particularly strong growth in biofuels. This sector did not exist in 1980 but experienced a steep rise during the 2000s to over one quadrillion Btu per year. This made biofuels a serious challenger to both wood and hydroelectric power, which both saw only limited growth overall. The former grew steadily between 1975 and 1985, but then slipped back to around its original level of 1. 8 quadrillion Btu. The latter began the period at the same level as wood but experienced more substantial growth. However, it also fell back to around 2 quadrillion Btu, with a particularly sharp drop in the late 1990s. Finally, wind power emerged late in the period but showed a gradual rise to around 0. 5 quadrillion Btu, suggesting that it, along with biofuels, will replace wood and hydroelectricity as the main sources of renewable every in the future.