The majority of the electricity your solar array will generate will be stored in batteries.
Batteries are your reservoir of energy for future use. It might help to compare this to water stored in a dam. Both have potential that can used for various purposes. Batteries are used to even out the power that your panels generate. They provide a constant load and 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.
See the list to the right for complete information about how to build your own solar-powered system.
Because solar energy is variable in its input parameters (clouds, night, etc), your batteries will probably not be topped off constantly. There are systems out there that convert 120-volt AC power to the lower voltage DC power your batteries want; this lets you bank power from your utility company for later use, and helps cut down on the wear and tear of your battery array, and can even recondition some types of batteries. When looking at battery chargers, talk to the manufacturer of the batteries you're using for their recommendations on charging voltage.
Batteries work by moving ions across a barrier via a chemical reaction. They need to be kept warm (roughly 10 to 25 degrees C, or broadly speaking, room temperature) and dry. The colder the battery is, the longer it'll take to charge, just like a car battery in the dead of winter. As with anything involving electronics, avoid extremes in temperatures; they can cause breakdowns in the chemical reactions that run your batteries. Since you're probably building your own battery enclosure, spending the money on high quality insulation is a good idea. Never let your batteries freeze.
You'll need to regularly monitor the storage capacity of your batteries, to know when to maintain them. Good battery care habits can more than triple the life of your battery system. You'll need a volt meter and hydrometer to do this; the first tells you how much voltage you're getting out of the battery, the second tells you the status of the electrolytes in the battery itself.
Your batteries are going to consume water; make sure that you fill them with distilled water to keep them running smoothly. You'll want to keep your batteries at a 50% charge state to maximize battery life, and you should keep your batteries electrolyte levels at the indicated level.
Because batteries run off of a chemical reaction, when they're charged, they can emit hydrogen and free oxygen. Keep your batteries in a well ventilated space, an away from open flames. Oxygen, in particular, is a primary source for contact corrosion. You'll want to inspect your battery contacts about once every six to eight months and clean off corrosion. Putting a layer of grease on the contacts will keep them from corroding as quickly.
As part of your maintenance progress, you should check your batteries for signs of aging. As they age, batteries get less efficient. If they're hooked up in parallel, their capacity drops. If they're hooked up in series, their voltage drops. Roughly the same time you check the contacts for corrosion and apply a new layer of grease to them, you should be running a capacitance meter and volt meter to them to check their overall performance.
Like your solar arrays, batteries can be wired in series to increase voltage, or in parallel to increase the storage capacity of the array. For wiring in series, the positive terminal is connected to the negative terminal – much the same way that you put batteries in a flashlight, and for the same reason. When hooking them up in parallel, you hook them up positive to positive and negative to negative.
Most of your home appliances want 12-volt current, and most commercial solar battery systems run with multiple 12-volt cells. Sometimes, for specialized needs, you'll need to run them in series for peak demand at 24 and 48 volt draws; again, knowing what your appliances need (covered earlier) is an important part of the process. Because batteries provide DC current (until you run through an inverter) and are hooked up to transfer DC between them, it's very important that your cells and the wiring that hooks them up be of the appropriate size and capacity; doing otherwise reduces efficiency, and can run the risk of electrical fires.
While we're talking about hooking batteries up to contacts, it's important that all your contacts get tightened evenly, and that the contacts be matched appropriately. You want to avoid differences in resistance in the contacts, as this will reduce charge to one battery string and reduce battery life. Also, put the main positive lead, and main negative lead on opposite corners of the array; this will even out any differentials in charge potential.
These are symptoms that batteries need replacement.
1) If the voltage rises rapidly when charging (you'll see this when the charger shuts down early), or drops rapidly under the load, this is a sign of battery wear.
2) Cell to cell voltage variations of 0.05V to 0.1 volts.
3) Increased water consumption.
4) Drops in overall specific gravity in the battery; this usually means that the solute has run out.
When you replace batteries, replace entire battery banks, rather than individual cells; putting a fresh sell in series or in parallel with one that's older will cause the newer battery to degrade down to the performance of the older one, rather than boost the older one up to new.