Hydroelectric power is an attractive source of relatively low cost electricity, which already provides about 20% of the worlds electricity. Hydroelectric systems with large reservoirs are particularly valuable for electricity grids that have large amounts of intermittent renewables because their reservoirs can buffer the variable component. However, only a small fraction of the worlds hydroelectric potential will be exploited because of environmental and social concerns about hydroelectric projects.
On the other side, small-scale hydropower facilities can play an important role in local energy economies. The global potential for small-scale hydropower, at capacities installed up to 2MW, is about 30% of the worlds hydropower potential. One draw back of small-scale hydropower is that it almost always involves run-of-river plants that lack reservoir capacity to store water.