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If an F-16 lost engine power, the hydrazine fueled emergency power unit (EPU) kicks in to provide enough electrical power to support the 3000 psi hydraulics system.
Emergency power unit - Wikipedia, the free encyclopedia
A UAV have no need for such powerful hydraulics simply because of the fact that it was not designed for high performance maneuverability. So even if a UAV is designed with hydraulics, the engineers would have battery backup at least long enough to allow the aircraft to either loiter to transmit off important data or to have a controlled landing.
Keyword search for you: 'uav glide ratio'. Plenty of information available.