What is the minimum required capacity for batteries supplying emergency lighting?

The minimum required capacity for batteries supplying emergency lighting is typically set at 1.5 hours. This duration ensures that in the event of a power failure, the emergency lighting systems can remain operational long enough to allow safe evacuation from buildings or facilities. The standard is based on the average time it would take for occupants to exit from various types of structures and to provide adequate lighting to facilitate that process, ensuring safety and compliance with building and safety codes.

The choice of 1.5 hours reflects a balance between providing sufficient backup power for emergency lighting while also considering the practical aspects of battery size, weight, and overall feasibility for installation. Longer durations, while providing more security, may not be necessary for many applications and can lead to increased costs and logistical challenges without significant benefit.