If it is a Level 1 (defined by NFPA and AHJ), then you need on-site storage too.
NFPA 110
5.1 Energy Sources.
5.1.1* The following energy sources shall be permitted to be used for the emergency power supply (EPS):
(1)* Liquid petroleum products at atmospheric pressure
(2) Liquefied petroleum gas (liquid or vapor withdrawal)
(3) Natural or synthetic gas
Exception: For Level 1 installations in locations where the probability of interruption of off-site fuel supplies is high, on-site storage of an alternate energy source sufficient to allow full output of the emergency power supply system (EPSS) to be delivered for the class specified shall be required, with the provision for automatic transfer from the primary energy source to the alternate energy source.
