Electrical power is the most critical resource for the International Space Station (ISS) because it allows the crew to live comfortably, to safely operate the station, and to perform scientific experiments. So, whether it is used to power the life support system, run a furnace that makes crystals, manage a computerized data network, or operate a centrifuge, electricity is essential. Since the only readily available source of energy for spacecraft is sunlight, NASA Glenn Research Center has pioneered, and continues to develop, technologies to efficiently convert solar energy to electric power. One method of harnessing this energy, called photovoltaics, uses purified silicon solar cells to directly convert light to electricity. Large numbers of cells are assembled in arrays to produce high power levels.

However, a spacecraft orbiting the Earth is not always in direct sunlight. Therefore, the ISS relies on nickel-hydrogen rechargeable batteries to provide continuous power during the "eclipse" part of the orbit. The batteries ensure that the station is never without power to sustain life-support systems and experiments. During the sunlit part of the orbit, the batteries are recharged. The process of collecting sunlight, converting it to electricity, and managing and distributing this electricity builds up excess heat that can damage spacecraft equipment. This heat must be eliminated for reliable operation of the space station in orbit. The ISS power system uses radiators to dissipate the heat away from the spacecraft. The radiators are shaded from sunlight and aligned toward the cold void of deep space.

The power management and distribution subsystem disburses power at 160 volts of direct current (abbreviated as "dc") around the station through a series of switches. These switches have built-in microprocessors that are controlled by software and are connected to a computer network running throughout the station. To meet operational requirements, dc-to-dc converter units step down and condition the voltage from 160 to 120 volts dc to form a secondary power system to service the loads. The converters also isolate the secondary system from the primary system and maintain uniform power quality throughout the station.

The International Space Station's electrical power system (EPS) will use eight photovoltaic solar arrays to convert sunlight to electricity. Each of the eight solar arrays will be 112 ft long by 39 ft wide. With all eight arrays installed, the complete Space Station is large enough to cover a football field. Because the Space Station needs very high power levels, the solar arrays will require more than 250,000 silicon solar cells. NASA has developed a method of mounting the solar arrays on a "blanket" that can be folded like an accordion for delivery to space. Once in orbit, astronauts will deploy the blankets to their full size. Gimbals will be used to rotate the arrays so that they face the Sun to provide maximum power to the Space Station.

The complete power system, consisting of U.S. and Russian hardware, will generate 110 kW (kilowatts) total power, about as much as 55 houses would typically use. Approximately 46 kW will be available for research activities.

Truss and Solar Array Assembly Sequence

Diagram of ISS Truss elements