Nearly every type power generating system also has a method of storing energy. Such systems, when tied to an energy grid, can generally accept excess power and give it back as needed. Systems not tied to a grid, however, require battery banks, either for full operation or in the case of an outage. Typically, battery banks will be charged by the solar array or wind turbines as they produce power, which is then disperse that energy as needed. Among the many types of batteries, the most commonly used is lead-acid. This technology uses lead plates and a sulfuric acid electrolyte either in fluid form, absorbed into fiberglass mats (known as AGM), or as a gel. The AGM and gel types of lead-acid batteries are collectively known as Valve Regulated Lead Avid (VRLA). VRLA batteries, when operated properly, are fully sealed, don’t emit gases, and don’t require water addition. These, along with other lead-acid batteries, are relatively inexpensive and widely available especially compared to other battery types.