Electrical engineers design emergency power systems by considering various factors such as the type of power failure, the amount of power required, the location of the system, and the availability of backup power sources. They determine the capacity and type of backup power source required, such as batteries, generators, or uninterruptible power supplies (UPS). They also design the electrical distribution systems to switch over to the backup power source in case of a power failure and ensure that the system can provide enough power to support critical systems during an emergency. They also test and verify the system to ensure it works correctly and can handle the required load.