Solar electric systems with battery storage are being used by businesses and residences all over the world to power a wide range of building equipment, lighting, appliances, computers and communications equipment. In general, these systems are best for applications in which backup power must be instantly available without interruption (for example, to power computers). They are also good for remote applications in which utility power is not available and a generator is not desirable.

A back-up or stand-alone AC solar electric system will usually have a PV array of ten or more modules, battery bank and one or more inverters. Two or more stackable inverters are an excellent choice for this type of system since they can work together to supply power to large loads and if one fails, the others can continue to operate at reduced output until repairs are made. The utility will back-up the solar and run the loads when available and needed. If utility power fails the power from the solar can run the backed up loads. A fossil fuel generator may be included to further back-up the system.

In most businesses and homes, an AC only system simplifies wiring by allowing the use of low cost, readily available switches, outlets, and fixtures. Savings on wire cost are significant, because the large gauge wire required for efficient transmission of low voltage DC power over long runs is avoided.Solar electric systems with batteries can be designed to power DC or AC equipment. An inverter is necessary for applications in which the DC power from a battery must be converted to AC. Although a small amount of energy is lost in converting DC to AC (typical inverter efficiencies are in the range of 90 to 95%), an inverter makes PV-generated electricity behave like utility power to operate everyday AC appliances, lights, and electrical equipment. Please note that you will need a special type of inverter if you want a battery backup system. For safety reasons most grid-tied inverters are designed to shutdown completely if there is a power failure.

Solar electric systems with batteries operate by connecting the PV modules to a battery, and the battery, in turn, to the load. During daylight hours, the PV modules charge the battery. The battery supplies power to the load whenever needed. A simple electrical device called a charge controller keeps the batteries charged properly and helps prolong their life by protecting them from overcharging or from being completely drained.

Batteries make solar electric systems useful in more situations, but also require maintenance. The batteries used in PV systems are similar to car batteries, but are designed for deep cycling use in which a larger percentage of the capacity of the battery is used each night (and then fully charged up each day). Batteries designed for PV projects pose the same risks and demand the same caution in handling and storage as automotive batteries. The fluid in unsealed batteries is highly corrosive, levels should be checked periodically, batteries must be appropriately ventilated, and batteries should be protected from extremely cold weather. In practice we have found that when properly maintained batteries last for about 5-8 years, after which their capacity is significantly diminished.

A solar generating system with batteries supplies electricity when it is needed. The amount of electricity that can be used after sunset or on cloudy days is determined by the output of the solar electric modules and the storage capacity of the battery bank. Including more modules and batteries increases system cost, so energy requirements (both in terms of peak loads and the average duration of the loads) are carefully studied to determine optimum system size. A well-designed system balances cost and convenience to meet the needs of the particular application, and can be expanded if those needs change. Nevertheless, battery backup PV systems are typically more expensive, and have higher maintenance costs than simpler grid-tied systems.