12 V and 24 V DC power sources are becoming increasingly popular compared to 24 V AC because it is easier to back-up DC with batteries. DC power systems can be directly connected to battery backup systems without the need for conversion, making the implementation simpler and more reliable during power outages.
This efficiency in backup capability is one of the primary reasons for the shift from AC to DC in many applications, including surveillance systems. [Reference: Axis Communications - Power over Ethernet for Network Cameras]