Complex sentence: Hydroelectric power is currently the largest producer of renewable power in the U.S. It produced around 66.8% of the total renewable power in the U.S. in 2008 which was 6.0% of the nation's total electricity.
Split into simpler sentences: Hydroelectric power is currently the largest producer of renewable power in the U.S. It produced around 66.8% of the total renewable power in the U.S. in 2008. Also in 2008, hydroelectric produced 6.0% of the nation's total electricity.