Thank you for your interest in Sump Pumps Direct. Each pump has it's own float switch. The float switch for the battery backup pump is set a few inches higher than the float switch for the main pump. When the main or primary pump is having difficulty keeping up with demand, the water level will rise higher insider your sump basin, thus setting off the secondary or battery backup pump to assist. This process will take place whenever you have AC power available to the system. In the event of a power outage, the battery backup pump, will obviously be your only working pump.