Bi-Modal Systems are grid tied but also have a battery bank in case the grid goes down and there is no power available from the electric utility. In these type of systems, the battery bank is not sized to run the entire house, rather a separate panel box is installed that house dedicated circuits such as refrigerator/freezer, well pump, lights and fans in certain rooms, television/radio,etc. As with the stand alone systems, these systems are more expensive than the Grid Tied Systems and required battery service and eventually replacement.
Below is a picture of a Sonnen battery, tastefully located in the main living area of the home.
In a PV System with Battery Back-up your solar modules will convert the sunlight to DC electrical current. A charge controller will then direct this energy either to batteries where it is stored to be used at another time, or to a central inverter where it is converted to AC current and then fed back through your breaker box to your utility provider. The energy you produce is then credited back to you on your electric bill.