The Emergency light is with built-in long-life and maintanance-free rechargeable Lead Acid battery. The Bulb has 3 modes:1. AC: Fit the bulb in regular B22 bulb holder, and it can be used as a regular bulb.2. DC- Bulb will illuminate as soon it is unplugged from socket/ Switched off. Thus as soon light goes out, the bulb illuminated automatically.