[sjburt]

For surveillance drones it's going to be wireless power. Either microwave or laser.

[TylerE]

Or solar, if the threat profile is low.

Could even combo the two approachs...have a large high altitude solar "mothership", that beams directed power at the drones.

[dreamcompiler]

PV is good enough for drones if wing area is large, the aerodynamics are super-efficient, and it's daytime.

[xchaotic]

Or 5G ;)