Scott is right that your best bet electrically is to have at least 20 battteries in series and avoid the losses from stepping up your voltage.
Frankly you are far better off directly heating the water in the sun and storing your hot water in an insulated tank. Just preheat your water over the roof before it goes into your electric water heater. Forty gallons of water on the roof costs much less than forty batteries and 4000 sq ft of photovoltaic cells and is a lot easier to install and maintain.
