For Comparison: A Commercial Emergency Power Unit
In a conventional backup generator, the generator must be rated at 1.5 times the expected peak power.
If an emergency power unit is to operate a load that requires 60 kW, the generator of the emergency power unit must be designed for 90 kW.
Even if this performance may only be needed for a very short time (a few minutes) or only once a day.
The rest of the day, the emergency power unit runs only in part-load operation, or even at idle. In these areas engines of emergency power units are very uneconomical, the need for fuel (mostly diesel or fuel oil) is high although little energy is supplied or needed. The generators of emergency generators are exposed to extreme load fluctuations and have to cope with this 24 hours a day. All of these factors enormously reduce the lifetime of an emergency generator. The waste heat generated by an emergency power generator can not be used at all and evaporates unnecessarily.
If a fault occurs within the emergency power unit, at the same time no energy is available at all.