EE WAS ON CAGE TO GO TO 850 LEVEL. A RAINSTORM IN PROGRESS DURING DESCENT. AT APPROX. THE 300 FT MARK, POWER SUPPLY TO HOIST WAS LOST DUE TO THE STORM. THE BRAKES ON HOIST WENT INTO A CONTROLL ED STOP. INSIDE THE WET CAGE EE LOST HIS BALANCE AND STRUCK WALL OF CAGE AND FELL TO FLOOR WITH SHOULDER, NECK AND HEAD PAIN.