Like they say up in New England, "Ya can't get there from here." kV by itself won't help you in calculating the Watts out. Maybe coupled with a half-dozen other parameters and at least as many assumptions, you could make something out of it... Truthfully, we don't often calculate Watts out because it's practically impossible to calculate accurately given the number of variables. Watts out can only be accurately measured, by a dynamometer. Normally we simply use Watts in, making rough assumptions as to the efficiency of the power system. Watts in is easy to calculate: Volts times Amps. This is hardly an exact science.

What good is kV by itself? It's useful for comparing motors of similar size and weight. A higher kV motor will spin a smaller prop faster, or will be "more powerful" given the same prop and battery, at the cost of higher Amp draw and shorter run time.

The "k" in kV stands for "konstant," either the German word for, or simply a play on the word "constant," as used in mathematical equations.