Each bucket on the water wheel is actually a "double bucket." Imagine two bowls with a little bit of one side cut out; the bowls are welded together on the cutouts so that a lip is formed in the middle. A nozzle is placed in the housing so the water hits the middle of the double bowl, on the central lip. The water makes a U-turn in the bowls, and exits opposite the flow from the nozzle.
Because the water makes a U-turn and is forced out opposite the flow from the nozzle, an impulse force is created. This force is the same as when you let go of an inflated balloon's nozzle and it flies off. Pelton's design takes advantage of two forces: The initial water pressure force coming out of the nozzle, and the impulse force created by having the water exit in a U-turn.
According to the Massachusetts Institute of Technology, Pelton's design achieved 90 percent efficiency. Students at the University of Iowa investigated Pelton turbines extensivel, and discovered that their turbine is about 80 percent efficient; the extra 20 percent was lost in bearing friction and backsplashing.
A Pelton turbine is very useful for small-scale hydroelectric generation. Because of its efficiency factor, it can extract almost all the power created by a small stream of water. According to the Houston Advanced Research Center, Pelton turbines are easy to make and impervious to debris clogging such as sand.