The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available. These systems help balance supply and demand by storing excess electricity from variable renewables such as solar and inflexible sources. Energy from wind, sunlight or other renewable energy is converted to potential energy for storage in devices such as electric batteries or higher-elevation water reservoirs. Storage technologies include pumped hydroelectric stations, compressed air energy storage and batteries, each offering different. Conversely, they could provide stored energy, possibly from their electric vehicle, during high-demand periods and receive direct billing rewards.