Abstract: A modularly attachable transfer switch assembly for operative connection to two alternate power sources and a power load is disclosed. The transfer switch can be quickly and easily installed upon existing electrical meters and therefore does not require modifications to existing electrical wiring systems. The transfer switch includes one power switch for selectively connecting a electrical utility power supply to the power load of a structure, such as a building for example, and a second power switch for selectively connecting an emergency power supply to the power load of the building. The transfer switch includes a mechanical interlock operatively connected to both the power switches for physically preventing both the power switches from assuming simultaneous ON positions, and a controller for controlling the emergency power supply and the transfer switches.