Hydroelectricity in Practice

The U.S. has 78 GW of hydroelectric capacity installed. In a year, these plants produce 272 TWh. Divide by 8766 hours in a year, and we find 0.031 TW (31 GW) of averagepower. This implies a 40% capacity factor. I was surprised by this: I thought dams churned along at a steady rate all the live-long day. Seasonal variations are apparently much larger than I appreciated—snow-melt being one factor. The following table lists all hydroelectric facilities in the U.S. bigger than 1 GW—representing 30% of total installed capacity in just 11 dams. The table shows each dam’s nameplate (peak) capacity, height, implied flow at peak generation capacity (after which spillways must be activated; assumes 90% efficiency), and capacity factor.