Comment Re:120 kW (Score 2) 22
The 120kW figure is indeed input power. Thrust is typically quoted in Newtons, not Watts. The input power is useful because it's a proxy for thrust and vastly easier to measure. Ultimately, none of that really matters, however: the real figure of merit for ion engines (all rockets, really) is Specific Impulse. When NASA claims these use 90% less mass for the same total impulse, they're saying it's about one order of magnitude more propellant-efficient than a chemical rocket.