[BrianG]

For the ESC? Current and voltage. An ESC is either on or off so the wattage dissipated by it is just the losses from the FET rdson, switching losses, and copper losses, therfore, very low. The voltage is limited by the various device ratings. But don't make the mistake of saying "if an ESC can take 6s and 100A, then it is rated for 2,200W".

Now, current and power are limiting factors for motors. The windings can only take so much current before they turn into a fuse. Motor efficiency and case surface area will determine how much power it can handle before it gets too hot.