A buck converter refers to a method of changing the voltage of a supply by regulating the output of a smoothed transformer secondary winding. You are "bucking" or kicking the voltage up or down. Another name is switching regulator or DC to DC supply or inverte/dropper. A solar PV panel generates power in the form of DC voltage and current. If it is fed into a buck converter it will produce a voltage regulated output within the power rating of the solar array and the buck converter. So if the panel produces 200watts at 25volts the input current will be 4amps and the output current will depend on the voltage regulated and its load - V^2/R=I^2*R=W. If the sun doesn't shine the power output will drop and the buck converter should maintain the voltage as long as V(in)>V(out)+V(drop). Similar sort of reasoning would apply to wind generation, though the supply would be from an alternator driven by the propeller, and hence AC.
