The vast majority of the electricity your solar array will generate will be stored in batteries; carrying the "electricity as water pressure" analogy further, batteries are your reservoir of energy for future use. They're used to even out the power load that your panels generate, and provide a constant load, and to give you power when the sun is down without drawing on your utility company. As mentioned earlier, your battery array will need to be hooked up to a controller, and probably an inverter if you want to use power for AC appliances.

Battery choices are fairly standardized, much the same way solar panels are. You'll want to make sure that you maintain your batteries carefully. For the most part, you're going to be using deep cycle (also known as "marine" batteries). These have lower peak yields per battery cell, but store the same amount of power. Their primary benefit is that they last longer under constant use.

You'll want as many batteries in a battery bank as you can get; the less your battery has to work, the longer it will last, and the best way to do this (even with deep cycle batteries) is to spread the work out.