While chemical propulsion systems store their energy in the propellants, the energy required by electric propulsion systems is generated by solar panels. The use of solar panels to supply power to an electric propulsion system is referred to as solar electric propulsion (SEP). Future missions, operating at high power levels or at great distances from the Sun will require an alternative source of power. If the safety concerns can be addressed, power could be provided by a nuclear electric power system, where heat from a reactor is used to produce electricity by direct thermoelectric or thermionic conversion using solid-state devices or by an indirect thermodynamic cycle.

Chemical propulsion is said to be "energy limited" because the chemical reactants have a fixed amount of energy per unit mass, which limits the achievable exhaust velocity or specific impulse. However, because the propellants are their own energy source, the rate at which energy can be supplied to the propellant (which is ultimately limited by reaction kinetics) is independent of the mass of propellant, so very high powers and thrust levels can be achieved.

Electric propulsion systems are not energy limited. Neglecting component lifetime considerations, an arbitrarily large amount of energy can be delivered (from a solar or nuclear power system) to a given mass of propellant so that the exhaust velocity (or specific impulse) can be much larger than that available from a chemical propulsion system. Electric propulsion systems are termed "power limited" because the rate at which energy from the external source is supplied to the propellant is limited by the mass available for the power system. This has the result of limiting the thrust of the electric propulsion system for a given spacecraft mass. Because of this, electric propulsion vehicles tend to be low thrust to mass ratio (and therefore low acceleration) vehicles.

Although electric propulsion vehicles have low thrust to mass ratios, they can have a larger total amount of impulse (the product of specific impulse and propellant mass, equal to the total change of momentum) than a chemical propulsion system. So, even though a chemical propulsion system can offer a high thrust to mass ratio, the propellant is expended in a short time at low specific impulse. By contrast, the low thrust to mass ratio electric propulsion system can be operated for periods ranging from hours to years and build up a larger total impulse.

	Why Use Electric Propulsion?
	Power for Electric Propulsion