What is the distinction between emergency generators, stored emergency power supply systems (SEPSS), and UPS, and when is one used versus the other?

Any examples are for illustrative purposes only. 

Emergency generator defined as a stationary device, driven by a reciprocating internal combustion engine or turbine that serves solely to drive the generator producing electrical energy whenever normal electrical supply is interrupted.

Stored energy emergency power supply systems (SEPSS) store energy in either of two basic forms.  Chemical energy in the form of storage batteries and mechanical or rotational energy by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system. 

The decision to use one type over the other is usually determined by the required time for the emergency power systems to deliver electrical power. Engine driven generators can provide as long as the fuel supply is maintained.  Hospitals with heavy electrical loads for critical care patient care requiring life support equipment, lighting, HVAC and other critical systems and the need to remain functional during uncertain emergency opt for the engine driven electrical generators.

SEPSS are typically used in smaller outpatient clinics, surgical centers and ambulatory facilities due to the lower acuity of the patients and that the duration that emergency power is required to be supplied is much shorter than an in-patient facility.  Emergency power is required to allow staff and patients to exit the facility, and to treatments or therapy in progress to be halted and evacuate the patients.  Runtimes for a SEPSS can be as short a few minutes to as long as 90 minutes.

In the event a battery powered UPS is used to “bridge” the 10 second gap from power interruption to generator start time, it would not be considered a SEPSS.

Additional Resources:
NFPA 111 – 2010: 8.3.1; 8.3.3; 8.3.4; 8.4.1