Editing Ion thruster (section)

== Energy efficiency ==
[[File:Average propulsive efficiency of rockets.png|thumb|upright=1.4|Plot of {{colorbox|#4a86e8}} instantaneous propulsive efficiency and {{colorbox|red}} overall efficiency for a vehicle accelerating from rest as percentages of the engine efficiency. Note that peak vehicle efficiency occurs at about 1.6 times exhaust velocity.]]

Ion thruster efficiency is the kinetic energy of the exhaust jet emitted per second divided by the electrical power into the device.

Overall system [[Efficient energy use|energy efficiency]] is determined by the [[propulsive efficiency]], which depends on vehicle speed and exhaust speed. Some thrusters can vary exhaust speed in operation, but all can be designed with different exhaust speeds. At the lower end of specific impulse, ''I''<sub>sp</sub>, the overall efficiency drops because ionization takes up a larger percentage energy and at the high end propulsive efficiency is reduced.

Optimal efficiencies and exhaust velocities for any given mission can be calculated to give minimum overall cost.