Solar photovoltaic modules instantaneously convert sunlight into electricity, and their power output varies directly with the amount of light striking the surface of the solar cells. Thus when the sun isn’t shining, like at nighttime for example, little or no power is produced by the solar array. This is fine for loads which only require power during daylight hours, however most loads require a continuous and reliable supply of electricity. This is limitation is overcome by using a bank of batteries to store the electricity as it is produced, and to supply power to the load when it is required.

There are a number of factors to consider when designing a battery system, and Generation PV engineers take these considerations into account to ensure a high level of system integrity, reliability and longevity.

• Load Matching - The most obvious requirement is that the battery system be properly sized to the requirements of the load. This also includes ensuring that the higher current required by some devices upon startup is accommodated.
• Days of Autonomy - Renewable energy systems are inherently variable. As such, the rate at which the batteries supply power may exceed the rate at which the batteries are being charged. The autonomy of the batteries refers to the length of time the batteries can support the load without receiving any input charge. Location and local weather patterns are major factors in determining the number of days of autonomy. For renewable power systems supporting critical loads, longer battery autonomies ensure the reliability of the system, in the case of unfavorable weather.
• Battery Configuration - Determining the proper configuration of batteries is essential, especially due to the fact that batteries come in a wide variety of voltages and amp-hour ratings. Batteries are wired in series strings to increase voltage and connected in parallel to increase amperage. Physical placement on a rack or in an enclosure must also be determined.
• Derating - Discharging a battery completely can damage it, and result in a reduced life span and decreased capacity. For this reason, the recommended depth-of-discharge to design for is 50-80%. In other words, the lowest state of charge your batteries should ever reach is 20%. Another reason to derate or design for a reduced battery capacity is temperature. In colder environments, the capacity of the battery is decreases with decreases in temperature.
• Cable Sizes - Even a perfectly designed battery system is in danger of failing if improper gauges of cable are used. Knowing what size of wire to use depends upon the distance the cable needs to span and the maximum current it will be required to handle.
• Installation – When designing medium to large sized battery banks, one must take into consideration the weight of the batteries and the installation’s geography. Batteries which feature high amp-hour ratings can easily weigh a couple hundred pounds each, making them quite inconvenient if not virtually impossible to manhandle and install in remote or hard-to-access sites.
• Maintenance – Different battery technologies require different amounts of maintenance to keep them running optimally. One technology may be favored over another when there are time, cost, and feasibility issues to deal with in terms of regular maintenance.