How do I size a battery bank?

Four inputs: the load in watts or amps, the required autonomy in hours or days, the maximum depth of discharge you will design to, and the derating factors for temperature and end-of-life capacity. Australian utility practice, expressed cleanly in Sydney Water’s HV specification, is the IEEE 485 method with a 10% design margin and a 25% ageing factor. For stand-alone power systems, AS/NZS 4509.2 gives the storage sizing method and autonomy guidance.