For several points of interest on earth (particularly plausible/notable/typical places) calculate the following for both ground level and an altitude of half a kilometer:
What is the minimum, average, and max wind velocity? For the average wind velocity what would be the pull of a kite with an area of a hundred square meters? What would be the weight of the kevlar necessary to hold down something producing that much force at the maximum wind velocity? After factoring in holding up all that kevlar, could a kite stay up at minimum wind speed? How much power could be generated by its remaining upwards force? How much of that would be lost in the electrical generator? How does that amount of power output compare to the per capita max and average power consumption in the United States?
My suspicion is that flying kites at an altitude of more than a few hundred meters is simply not worth it, and that the optimal kite size in practice is around a thousand square meters, and that in an area where the wind never dies down a kite system compares quite favorably to a turbine system, once you get all the engineering problems worked out.
Also, I have a dumb question about controls - At what altitude do the sorts of control systems used for power kites simply stop working due to even very tensioned materials having a lot of slop at that length?