The vast majority of UAVs for public and civil use will be powered by advanced battery technologies.
There will be some fuel engine based UAV technologies but they will be expensive, heavily regulated and application specific.
At least ninety percent of public, hobby and civilian UAVs will be powered by some kind of battery.
Nickle Cadmium (NiCad) and Nickle Metal Hydrid (NiMH) were first used, but Lithium based batteries have now taken over.
The reason for the popularity of lithium batteries is simple - really high energy density.
Lithium batteries have enough stored energy to let our UAVs fly for at least a useful length of time.