Deep cycle batteries are, without a doubt, one of the most fragile but critical components in an off-grid renewable energy system. A grid-tie system will not be at the mercy of batteries since the commercial grid conveniently acts as your storage, that is as long as the grid is functioning. Some grid-tie systems might actually have a battery bank to allow the system to continue supplying AC power to the house or business in the event of a commercial power interruption.
Because of their function in either case, we typically look at batteries as the renewable energy fuel tank. Power can be drawn out for household needs when there is no renewable or commercial power input. This is basically how a grid-tie system works. When connected to the commercial grid, your AC/DC interface is different from the stand alone system. When grid-tied, you are not really dependent on what your renewables are producing since you can draw off the grid any amount of power you need, when you need it.
On the other hand, with an off-grid system batteries are a pure necessity. Your renewable energy input (solar, wind, hydro, etc,) will most likely be DC or rectified "wild AC" and designed to connect straight to a battery bank. As a result, the inverter doesn't draw power directly from your energy input, but rather from the batteries. This makes the batteries not only a storage system but a buffer between DC input and AC output.
Bank of L16 flooded deep cycle Batteries.
Therefore, the battery has an important job and should be approached as such. In reality, this is the one place that we tend to want to scrimp and save with an alternative energy system, but the opposite is usually true. The good news is that you don't have to empty your wallet just to build a sufficient battery bank. You will need to adopt a realistic attitude about what your storage needs will be and what is available in your area. You will normally have a variety of options when it comes to different battery types and styles as well as storage capacity.
One thing to keep in mind is that too small of a battery bank can actually be expensive in the long run but too big of a bank may be problematic as well. Understanding what the batteries suitable for a renewable energy system have to offer is needed for building a proper battery bank. Understanding how to care for them is key to their longevity. In any case, the technology developed over a century ago is going to play a big part in an off-grid renewable energy system.
This will depend on a couple of factors.
One factor is the amount of power you produce from renewable energy sources. You want your power production to be able to fully charge them at times. On the other hand if the battery bank is too small, they can be prone to charging too rapidly and also not having any meaningful storage capacity.
Second is the up front expense you either can or want to incur. There is some flexibility in the storage capacity previously mentioned, but the biggest cost factor is the type of deep cycle battery. For a given amp hour capacity, you have choices that can span thousands of dollars. This point needs careful attention so you will not fall into a common trap that can be set by unwary installers or uninformed resellers.
Your budget may keep you from installing the most cost effective batteries initially. This is what we call a "Training" set. It will provide you with an inexpensive way to get the feel for taking care of your batteries. At the same time you also won't feel too bad if accidentally ruin them.
Photo Courtesy of DOE/NREL - Credit, Warren Gretz.