Without batteries, you will only be able to use solar power when the sun is shining. Honestly that is not so convenient so I would say batteries are a necessity with a solar panel system. Of all the battery choices (wet cell, maintenance free, lithium, Tesla, salt water, etc), I only have experience with wet cell and maintenance free batteries. Therefore, I will limit my discussion to these two types of batteries. These are also the most common types used for solar power.
Most of us own or have owned a car; a battery is used to start the car and power parts of the car during use. Typically this is a wet cell or maintenance free battery. These batteries work great because they are given a continuous charge during use. However, once you turn off the engine, it does not take long for these batteries to drain. Another type of battery, common in golf carts, sailboats and electric wheel chairs is deep cycle batteries. These batteries are designed to be charged and discharged repeatedly; however they are not designed to be discharged completely. It is best if deep cycle batteries are not discharged below 40-50%. Typical life of a car battery is 3-4 years. Typical life of a well cared for deep cycle battery is 7-10 years. Typical life for a poorly cared for deep cycle battery is 1-3 years; they are much to expensive not to care for properly. Batteries will be about 1/3 (or greater) of your solar budget.
When I lived aboard a sailboat, I had wet cell golf cart batteries. I used two 6 volt Trogan T105 wired together in series to make a 12V battery bank. These were strong reliable batteries but it was a pain because wet cell batteries give off small amounts of hydrogen gas in addition to the possibility of sulfuric acid spills. Wet cell batteries need to be vented and also maintained (water added). I prefer to have my batteries inside my home and I did not want to vent them to the outside as that opened up another hole in my structure. Maintenance free, for me, are a better choice.
Deep cycle batteries are rated in amp hours; this is like the storage capacity of a water tank. Amp hours is the amount of energy the battery will store if fully-charged. For example, my AGM batteries are 125 amp hours each. As previously stated, you should only discharge a deep cycle battery approximately 50%; therefore an 125 amp hour battery is only really providing 62.5 amp hours of energy for use before needing to be completely recharged.
In part one of this series on solar power, I mentioned this series of blog articles would be for a basic solar setup costing in the neighborhood of $3500-5000. This size setup can easily accommodate 4-6 AGM batteries. I have experience with both the Vmax 125 amp hr and the 155 amp hr batteries. Both are excellent. When combined (wired in parallel so it remains 12V but the amp hours are additive) into a system, four 125 amp hr batteries will provide 500 amp hr (125 x 4 = 500) of storage or 250 amp hr (50% of 500) of usage before recharging.
Shown below is my battery storage, two batteries on each shelf and enclosed in a cabinet that becomes a lamp stand next to my chair. This is the advantage of AGM batteries, they do not need maintenance so can be stacked.
In the next installment of this series on solar power, I will discuss the inverter. The inverter converts the battery power to AC electrical current for use in typical appliances and other needs in your home.
Below are affiliate links for the batteries discussed in this article. As always, questions are welcome.