Batteries for off-grid power systems are expensive. Getting the maximum life expectancy out of them will reduce the cost of the system over its lifetime. In the past buying all the battery capacity you could afford was considered the right decision. Today better advice may be to buy all the battery capacity you can afford to charge properly.
Deep cycle flooded lead-acid batteries have a cycle life related to Depth of Discharge (DOD). The amp-hour rating is usually stated for a 20 hour rate; however most manufacturers state two or more discharge rates. The manufacturer’s data sheet should tell you all you need to know about the battery, if it doesn’t ask them for more information or choose another manufacturer. Here is a link to a Trojan L16RE-A data sheet for an example. Some things to note is the amp-hour capacity is at a temperature of 80°F (27°C) and an end discharge voltage above 1.75 V/cell. There are also charts for the cycle life by depth of discharge and available battery capacity by temperature. 1.75 V/cell end discharge is not a good choice for off-grid power systems if you desire a higher number of discharge cycles. As an example the Trojan L16RE-A data sheet at the link above shows about 800 cycles at 100% DOD, 1000 cycles at 80% DOD and 1600 cycles at 50% DOD.
Off-grid solar power systems rely on opportunity charging when sunlight is available. Sizing the battery to have a lower depth of discharge to increase cycle life requires a larger battery to provide the power necessary for the load at the desired DOD. To increase availability when there is no sun (bad weather) the size of the battery must also be increased to account for the number of days desired. The increased availability may result in a battery bank that operates at a high state of charge (SOC), perhaps 80% or above and may only be discharged further during longer periods of bad weather which the system was designed for. Charging efficiency is usually stated as approximately 85%, however it is not linear and charging a battery from an incremental (SOC) of 80% is less than 60% efficient. This results in nearly half the available charging energy being lost due to the lower efficiency when charging from 80% SOC to full charge. If solar is the only source of power in an off-grid system, there may not be enough hours of sunlight to charge the battery, additional power will also be needed for equalizing the battery. The options are to increase the size of the solar array or use an additional source of power such as a generator. For good service life the manufacturers’ procedures for installation, charging and maintenance must be followed. It is also important to keep a record of battery maintenance for warranty purposes.