Variable renewable energy is a renewable energy source that is non-dispatchable due to its fluctuating nature, like wind power and solar power, as opposed to a controllable renewable energy source such as hydroelectricity, or biomass, or a relatively constant source such as geothermal power or run-of-the-river hydroelectricity. Critics of wind and solar power warn of their variable output, but many studies have shown that the grid can cope, and it is doing so in Denmark and Spain.