Personally, if you are wanting your energy storage for a full day of back up power, I always recommend looking at the day (eg., time of year) when your solar system is producing the lowest amount of energy. Then, based on the amount of solar your system is producing on that day, use this formula:
Energy Storage (kWh) = Daily Energy Consumption - Daily Solar Generation
Reccommend doing consults with reputtable solar contractors in your area and then doing a comparison. Ask about Franklin batteries, which I think are the best for home storage options. Good luck!
