Renewable generation sources such as solar and wind energy are variable by nature. Because of this, some people believe dedicated energy storage is needed to support solar and wind power. But this isn't the case at transmission scales.
Historically, variations on the electric power system (caused by changes in system load, generation and dispatch, and network topology) have been handled at the system level because it is less expensive to aggregate variability before balancing it. As a result, storage is almost never "coupled" with a single energy source; it is most economic when it is operated to support an entire system.
Storage is nearly always beneficial to the electric power system. This is why 20 gigawatts of pumped hydropower storage was built in the United States decades before wind and solar energy were considered viable generation technologies. Even with the addition of relatively high penetrations of solar and wind generation, energy storage would continue to provide support to the entire grid—storing energy from a mix of sources and responding to variations in net demand.
Today, more than 26 gigawatts of wind power are operating in the United States, and no additional storage has been added to the electric power system as dedicated support. More than a dozen studies have analyzed the costs of large-scale grid integration of wind power and found that no additional storage is necessary to integrate penetrations of up to 20% wind energy in large balancing authority areas.