Significant factors in the selection of a nonpropulsive power system are the mission, power requirements, mass, size, ease of integration, compatibility with the space environment, reliability, and availability.
Through 1970, one-day missions will probably use batteries. Since longer missions may require a trade-off of mass for longer use time, fuel cells can be employed. Photovoltaic generators are suitable for power requirements below 10 kw. Larger power requirements and extended missions will require the further development of mechanical, thermoelectric, thermionic, and isotopic generators.