1). While the line may have to be fused for 20a/240V it probably doesn't draw that much. It is however describing the outlet that is most common for 240V devices in the USA
(The horizontal blade outlet is only 15 amps and is fairly rare).
2.) just because it says 20a/240V doesn't mean it actually draws 4800 watts. Big power supplies are notorious for low power factors (meaning the current is out of phase with the voltage). This is why elecrical gear tends to be rated in VA rather than watts, and why the electric company will charge you extra for low Power factor loads. VA is amps x volts. However if you remember calculus, if you make the current and voltage 90 degrees apart, energy is the integral of power over time the integral of sin(x)*cos(x) dx over appreciable periods of time is in fact ZERO! The ratio of power consumed/VA is called power factor.
While it might draw 20 amps, it will be far enough out of phase with the current, that the actual power is probably less than half that. That still means there is upwards of 2000 watts to dissipate, although often startup power is a significant multiple of run power, so the actual power dissipation may be a lot lower. In any case, the efficiency of these devices tends to be unimpressive. The real reason that we are still using guided missiles and bombs instead of laser death rays, is a guided missile or bomb is a far more efficient way to deliver energy to a target.