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Bi-modal systems are a hybrid of a grid-tied system with an off-grid battery based system. They are able to play either roll. However they are usually designed with the assumption that the grid is available most of the time. Therefore battery banks are usually sized for "survival mode" for economic reasons. The grid itself is the real battery for these systems.
These systems require additional components and special inverters that automatically handle all the different power scenarios. They can be outfitted with a generator and automatic generator start for a "if-all-else-fails" scenario. Therefore the cost is about twice the cost of a grid-tied only system.
With the grid being available, the system acts like a regular grid-tied system. It will generate power from the sun and power the appliances of your house. Extra power will be sent to the grid and make the Utility meter spin backwards. When power is need it will be drawn from the power grid and the meter will spin forwards. The batteries remain always fully charged.
If the power grid is not available due to an outage, the system will instantaneously power the loads in your house from the battery storage or from the solar panels or both. A built-in transfer switch will disconnect the house from the Utility grid. If the Utility grid is restored and stable the inverter will reconnect the house loads to the grid. The batteries will be recharged from the sun or the grid.
In the unlikely but possible case that the grid is not available and the batteries have been depleted, the system can then either manually or automatically be charged from a generator. The generator will power the house and charge the batteries.
These systems are eligible for all the incentives that regular grid tied systems are eligible for.
Depending on battery storage size these systems can be twice the cost of a regular grid tied system.