Batteries are your storage medium for the energy that you harvest from the sun. A larger battery bank means you can store more energy which is useful if you have long periods without sunny days, or if you have a lot of high power equipment that is used regularly. Batteries come in different voltages, which are multiples of 2, since all battery cells have a nominal voltage of 2V. So you will find 12V, 6V, 4v and 2v batteries.

Joining batteries or battery cells together in series (positive to negative along a line) adds up the voltage. Joining them in parallel (positive to positive and negative to negative) adds up the storage capacity of the batteries, but the voltage remains the same. Larger systems typically have smaller voltage batteries (2V or 4V) joined together in series to make 24V or 48V systems. This is really due to the manoeuvrability of the individual cells, since a high capacity 24V battery on its own would weigh about 1 tonne, so, the 12 x 2V cells are separated to enable them to be installed without a forklift truck!