What is the minimum required on-site fuel supply necessary?

Any examples are for illustrative purposes only. 
"Basic Care" patient rooms in facilities, such as those used for inpatient behavioral health, do not require an EES.  However, in many of these facilities, the generator is the alternate source of power for the illumination of the means of egress, emergency (task) lighting, exit lights, and/or the fire alarm system. NFPA 101 Life Safety Code requires these all to have a minimum duration of 1-1/2 hours (Class 1.5) (which may also be from a battery source).

The Joint Commission Emergency Management Standard requires that hospitals plan for managing its resources and assets describing in writing the actions that will be taken to sustain the needs of the hospital for up to 96 hours based on calculations of current resource consumptions. The facility should assess how it would be affected if outside emergency support could not be obtained for 96 hours.  This does not mean that they need to have 96 hours worth of fuel on site.  The plan could include memoranda of understanding (MOUs) with suppliers to replenish fuel as needed during the emergency period.  Additionally, the plan could be to operate without normal branch of power to reduce fuel consumption, to extend run-time of the available fuel. If the generator is used as the backup power source for the life safety branch of the electrical system, the facility should have enough fuel to run the generator for a least 1-1/2 hours for as long as the building is occupied.