The electricity created by your solar energy system has to be stored somewhere. Grid-tied systems store their power on the grid but off-grid systems require a bank of batteries.
There are two basic types of batteries: lead-acid and lithium-ion. Lead-acid batteries are larger and heavier but also less expensive. Lithium-ion batteries are lighter, smaller, and generally more durable than lead-acid batteries because they can be charged and discharged to greater extremes. If space for the batteries is limited, lithium-ion is your choice.
When calculating the number of batteries needed, consider these factors: storage capacity (total amount of electricity the batteries can hold), maximum charge rate (how much electricity the array produces), maximum discharge rate (the greatest amount of electricity your household will need at any one time), and the temperature extremes the batteries will be subject to. Finally, the amount of energy a battery stores is expressed most often as amp-hours (Ah).