The International Space Station runs on solar power. Eight large solar arrays, combined with newer rollout arrays installed over the past few years, generate between 84 and 120 kilowatts of electricity. That’s enough to power more than 40 average homes, all from sunlight collected in low Earth orbit.
The Solar Arrays
The station’s primary power comes from four sets of solar wings mounted on a long truss structure that stretches across the station like a backbone. Each set contains two wings, giving the station eight arrays total. These arrays use silicon-based photovoltaic cells to convert sunlight directly into electricity. As the station orbits Earth every 90 minutes, it spends roughly 35 of those minutes in Earth’s shadow, so the arrays need to produce enough power during the sunlit portion to both run the station and charge its batteries for the dark periods.
Starting in 2021, NASA began installing a new generation of compact, rollout solar arrays (called iROSA) that mount on top of the aging original panels. Six of these newer arrays have been deployed in pairs. Each pair produces more than 20 kilowatts of electricity and boosts the station’s overall power production by about 30%. These upgrades were necessary because the original arrays, some of which have been in space for over two decades, have gradually lost efficiency from exposure to micrometeorites, radiation, and the harsh thermal cycling of orbit.
Battery Storage for the Dark Side of Each Orbit
Since the station passes through Earth’s shadow on every orbit, it needs stored energy to keep running during those roughly 35-minute blackout periods. The station originally used nickel-hydrogen batteries, but these were replaced with lithium-ion batteries starting in 2017. The lithium-ion units are lighter, more compact, and last longer.
Each battery started with a capacity around 108 to 113 amp-hours and has held up remarkably well. After more than 10,500 orbital cycles (each cycle being one trip around Earth), the batteries showed only modest capacity loss, dropping just a few amp-hours from their initial ratings. That durability is critical because swapping out batteries in space requires complex spacewalks and costly resupply missions.
How Power Gets Distributed
Generating electricity is only half the challenge. The station also needs a way to deliver clean, stable power to every module, experiment rack, life support system, and laptop on board. The U.S. segment uses a two-tier electrical system: a primary network that carries power at a variable 128 to 173 volts, and a tightly regulated secondary network that delivers a steady 120 volts to equipment and experiments.
The bridge between these two tiers is a set of converter units that step the fluctuating voltage from the solar arrays down to a constant 124.5 volts (plus or minus 1.5 volts). These converters handle loads ranging from less than 1 amp during light use to over 50 amps at full draw, and they operate at about 95% efficiency. They also include built-in safety shutoffs: if voltage spikes above 173 volts, if current exceeds safe limits, or if a short circuit occurs, the converter cuts power to protect downstream equipment.
The Russian segment of the station operates its own separate 28.5-volt power system inherited from earlier spacecraft designs. To share power between the two segments, adapter units convert Russian 28.5-volt power to 123 volts for use on the U.S. side, and vice versa. This interoperability means the station can route power where it’s needed most, regardless of which segment generated it.
Why the Station Uses DC Instead of AC
Everything on the station runs on direct current (DC), not the alternating current (AC) you’d find in a home outlet. Solar panels naturally produce DC power, and batteries store and discharge DC power, so using DC throughout the station avoids the weight and complexity of AC conversion equipment. The 120-volt secondary system is close enough to household voltage that many standard electronic components can be adapted for use on board, which simplifies the design of scientific instruments and crew equipment.
Keeping the Arrays Pointed at the Sun
The solar arrays don’t just sit in a fixed position. Each wing is mounted on a joint that rotates to track the sun as the station moves through its orbit. This tracking system, called the Solar Alpha Rotary Joint, continuously adjusts the angle of the arrays to maximize the amount of sunlight hitting the cells. Without active tracking, power output would drop significantly every time the station’s orientation shifted relative to the sun. A second set of joints, called Beta Gimbal Assemblies, provides a second axis of rotation for finer adjustments.
This combination of sun-tracking hardware, high-capacity batteries, efficient power converters, and newer rollout arrays keeps the station running around the clock, 250 miles above Earth, on nothing but sunlight.