Hydroelectric plants convert the energy of moving water into electricity. On average, hydropower ranks among the energy sources with the lowest levels of greenhouse gas emissions per unit of energy produced, but levels of emissions vary enormously between projects. In 2019, hydropower supplied 16% of the world's electricity, down from a high of nearly 20% in the mid-to-late 20th century. It produced 60% of electricity in Canada and nearly 80% in Brazil. Reservoir-based hydropower plants provide a highly flexible, dispatchable electricity supply. They can be combined with wind and solar power to provide peak load and to compensate when wind and sun are less available.