Deep Cycle Batteries are a key component in a stand-alone renewable energy system. Without deep cycle batteries, you can only use power at the time you produce it (i.e. you will not have power when the sun isn't out if you don't have batteries in your solar electric system).
In renewable energy systems, deep cycle batteries provide the energy storage for your system. Unlike your car battery, deep cycle batteries that are used in renewable energy applications are meant to be discharged and recharged (cycled) repeatedly. To maintain healthy batteries and prolong battery life, most manufacturers suggest limiting the depth of discharge to about 20%. (That means the deep cycle batteries will be at 80% capacity or better.) At the very least, do not allow the batteries to be discharged below 50% Depth of Discharge (DOD). Often an inverter will have a Low Voltage Disconnect feature that will disconnect loads at a given set point. Low voltage alarms can provide audible warnings as well.