for battery bank calculation, first u hav to know about ur daily need of electrical energy in kwh. the peak load or maximum wattage of appliances in ur house to opt for suitable inverter, and the days of autonomy, battery bank capacity accordingly and ultimately the no. of batteries after calculating the above needed data. the daily energy use is multiplied by the factor of 1.5 which has been taken for all the losses when there will b an increase in temperature.