Many renewable energy systems that are not connected to the power grid have some method of storing energy. Energy storage allows a system to operate even when there is no input or when the input is not capable of meeting all of the demands of the loads. Batteries Most stand-alone systems use rechargeable batteries for energy storage. Batteries used with RE systems are often deep cycle batteries, meaning they can discharge a significant amount of the energy they store before requiring recharging.
Some systems also use cheaper shallow cycle models.
A lead-acid battery. Courtesy of Solar Energy International . The frequency and depth of energy discharge from a battery has a significant impact on how long it lasts. For instance, frequently discharging a battery below the level recommended by the manufacturer might reduce the lifetime from five years to one year. Batteries convert electricity to stored chemical energy in the charging process, and chemical energy to electricity in the discharge process.