If you are going to install a grid-connected solar electric system with a battery bank or a stand-alone system you’ll need batteries. The more energy your home consumes and the longer the cloudy spells, the more batteries you’ll need. (As a friendly reminder: because batteries are expensive and require periodic maintenance, be sure to cut your electrical demand through efficiency and other measures first. Efficiency will save you a fortune on PV modules and batteries!)
 
Most batteries used in solar electric systems are 6-volt, deep-cycle lead acid batteries. Trojan L-16s have been the mainstay of the solar electric industry for years, but their dominance in the battery market has been challenged in recent years by Surrette Batteries, among others.
 
Batteries are wired in a combination of series and parallel circuits to produce 12-, 24-, and 48- volt systems.As you will soon see, batteries are rated by their capacity to store electricity.The common measure for battery storage is amp-hours. An amp-hour is one amp of current flowing for one hour.  Just to give you an idea of what you will be looking for,a battery in a solar electric system should probably store over 350 amp-hours of electricity to be useful.
 
Most deep-cycle lead acid batteries manufactured for solar systems are pretty good. But be sure to check out the storage capacity and manufacturer warranties first. Surrette S460s come with a seven-year warranty. The manufacturer will replace the battery free of charge for the first two years if it fails during that period. After that time period, the manufacturer will replace it at a prorated value.
 
Although lead acid batteries are less efficient than some of the newer battery technologies on the market today, old batteries are recycled. In fact,nearly 100 percent of the lead from used batteries makes its way back into the production cycle.
 
Solar electric systems can run on ordinary car batteries, but not for long. Car batteries are not designed for deep discharging—drawing off lots of power. They’re designed to crank out tons of amps to start a car, but they’re the rabbits of the battery world. What you need is a tortoise, a battery that can give you all it has for long periods of time. No sprinters need apply.
 
So be sure not to make the mistake of running a solar electric system on car batteries. It is a waste of your time and money.

Also, be sure not to purchase marine deep-cycle batteries. They are only slightly better than car batteries. You’d be lucky to get more than a year or two of service out of this type of battery in a stand-alone solar electric system. Both car and marine batteries are manufactured to optimize cranking power, that is, they are manufactured with thin plates to provide a surge of power to start engines.  Thin plates, however, are damaged by the deep discharges that typically occur in solar electric systems.
 
Golf cart and forklift batteries make a better choice, as they contain many thick lead plates capable of undergoing deep discharges day after day.  ....read more