Battery bank sizing is critical. Under sizing the battery bank will reduce the system usability and over sizing it will dramatically impact on the cost.

To size a battery bank, what you need to come back to the energy needs as determined after the load assessment and convert the energy needs from Watt hour to Amp-hour. Amp-hour is energy measure for batteries. To convert Watt hour into Amps hours, you just need to divide watt hours by system voltage.

Again, you will need to de-rate energy needs in amp-hour, for inverter efficiency and battery depth of discharge. Depth of discharge or DOD is a percentage representing the usable capacity of a battery. It varies from 50 to 80% depending of the manufacturer and type of battery.

It might happen that the sun is not fully available for a string of days. To prevent power outage, the battery bank should be oversized to provide power despite absence of the sun. In solar language, Days of autonomy is the number of days a system can supply power without being recharged.

The final battery bank configuration will depend on the capacity or the amps hour of the batteries available on the market.

In designing a battery bank, you will divide the overhaul battery bank size obtained after all adjustments and days of autonomy over sizing by the amp hour size of the available batteries. The number of batteries you get will be adjusted to take into account system voltage: batteries voltage comes by multiple of two and the largest batteries are of 12V.