In some cases battery banks are an optional component of your residential solar or wind power system, while in others they are necessary. In either case, the bank’s function is to store energy for later use. In an off-grid system, the battery is necessary because solar power is your only source of energy, and you don’t stop needing power when the sun goes down. Your system will collect excess solar energy while the sun shines and store it in a battery bank until you need to use it.
In an on-grid system battery banks may still be useful, because they will allow you to store solar energy for use when neither the sun, wind or your traditional power source is available -for instance, if there were a power outage at night.

Picture from backwoodshome.com
For more information on battery banks, please visit:

http://www.backwoodshome.com/articles2/yago88.html Solar Power 101 with Backwoods Magazine – an excellent in-depth look at the specifics of solar power battery banks