Batteries are used with solar PV systems for one of two reasons; they furnish a source of backup power in case grid power goes away or they can furnish 100% of the power available for an off-grid home.
There are many reasons to want backup electric power, power without interruption. But to achieve that convenience the cost will be high. Most of us can not remember when the last time we were forced to do without electricity because it didn’t last for a long time. Long power outages are actually fairly rare but they can certainly be memorable when they do occur. And losing power for several hours, while an inconvenient, is usually nothing more than that. An inconvenience.
For someone wishing to live off-grid, the situation is significantly different. The batteries are critical 100% of the time. And you must be willing to perform basic battery maintenance to assure maximum performance. The process is not difficult just necessary.
However, current battery technology will allow you to use sealed batterys that require virtually no maintenance. The tradeoff for this convenience is a higher upfront cost and more critical charge/discharge regimen.
Traditional "flooded" batteries will require regular maintenance but will generally outperform sealed batteries. And their cost/capacity is lower than their sealed counterparts. Regardless of your choice, batteries can be dangerous if not treated with extreme care. They store a tremendous amount of energy, create explosive hydrogen gas during charge and discharge cycles, and contain dangerous chemicals.
The weight of a battery is a clue to its energy storage capacity. Lead is a primary component of a battery and the more lead the greater the capacity. Deep-cycle batteries are designed for regular charge-discharge use typical to the demands of off-grid life and therefore contain more lead.
However the larger the battery bank the more power will be needed to fully recharge it. And keeping the battery at a full state of charge is a primary requirement to maximize its life. Chronic undercharging will reduce the batteries capacity quickly while overcharging it will stress the battery internally. Both practices will shorten the batteries life.
Most first time off-grid PV system owners are advised to buy a cheaper "starter" battery bank to practice on since there really is an art to recognizing and following the correct maintenance procedure. If you’re a fast learner you can always upgrade to a high end battery bank after a year or two and be able to sell the starter set sooner while it’s still got a good amount of life left in it. Many solar PV integrators will offer a battery maintenance option to assure proper care is given the batteries. It’s money well spent.