Space-Based Solar Power: How It Works and Where It Stands

Key takeaways

• Above the atmosphere sunlight delivers about 1,361 W/m², against a standard peak of 1,000 W/m² at the ground. The bigger gain is time: a geostationary satellite is in sunlight for about 99% of the year.
• A space solar power station would convert sunlight to electricity, then to microwaves, and beam them to a rectenna on the ground that turns them back into electricity.
• Caltech’s demonstrator beamed detectable power from orbit to Earth in 2023. No system has delivered useful power to a grid.
• ESA-commissioned studies in 2022 found a possible case by 2040, while NASA’s 2024 study found space solar more expensive than terrestrial alternatives. Launch and manufacturing cost decide it.

Solar panels on the ground stop at night, lose output under clouds and see sunlight filtered by the atmosphere. A collector in orbit avoids all three. The idea of space-based solar power is to gather sunlight there and send the energy down as a beam, giving a renewable source that runs almost around the clock. The physics works; the open questions are scale and cost.

How much more sunlight is there in space?

At Earth’s distance from the Sun, sunlight carries about 1,361 W/m² before it enters the atmosphere (Kopp and Lean, 2011). At the surface, the reference spectrum used to rate solar panels integrates to 1,000 W/m² on a clear day with the Sun high (ASTM G173 AM1.5 Global, via PVEducation). So the intensity gain from going to orbit is about a third, not tenfold.

The large gain is in hours. A geostationary satellite passes through Earth’s shadow only in two eclipse seasons around the equinoxes, for at most about 72 minutes a day (NOAA GOES eclipse schedule). Added up, that is roughly 1% of the year, a little over 80 hours. A ground panel, by contrast, produces nothing at night and little in winter or under cloud.

How the energy gets to the ground

Most designs place large photovoltaic arrays in geostationary orbit, about 35,786 km above the equator, where a satellite stays fixed over one point on the ground. The electricity drives a phased array of microwave transmitters. Steering the beam electronically, by adjusting the phase of each element, keeps it pointed at a receiving antenna on the ground called a rectenna, which converts the microwaves back into direct current.

Microwaves are favoured because they pass through cloud and rain with little loss. The beam must stay wide to keep its intensity low and safe, so the receiving site is large: designs studied for Europe deliver gigawatts per satellite through rectennas several kilometres across. Lasers would allow much smaller receivers but are blocked by cloud and raise their own safety questions.

What has actually been demonstrated

In January 2023 Caltech launched its Space Solar Power Demonstrator. Its MAPLE experiment, an array of lightweight microwave transmitters, steered a beam onto receivers in space and sent a detectable signal to a receiver on Caltech’s campus. Caltech announced the result on 1 June 2023 (Caltech). The power involved was tiny; the result showed that a light, flexible, electronically steered array can work in orbit.

In November 2022 ESA approved SOLARIS, a preparatory programme to study the technology ahead of a possible decision on a full development programme in 2025 (ESA).

Is it worth the cost?

The two studies ESA commissioned in 2022, from Frazer-Nash and Roland Berger, concluded that space solar could supply competitively priced electricity in Europe by 2040 if the technology matures. Their base case of 54 satellites by 2070 put costs at €418 billion against €601 billion of benefits, mostly avoided terrestrial generation and avoided CO₂ (ESA). NASA’s Office of Technology, Policy and Strategy reached a harder conclusion in January 2024: the two designs it assessed would be more expensive than terrestrial alternatives, though costs could fall if capability gaps are closed (NASA).

The disagreement comes down to assumptions about launch prices, the mass of the arrays and how much of the assembly robots can do in orbit. Cheaper reusable launchers help, but a single gigawatt-class satellite would still be far larger than anything built in space so far.

For a reader who follows electricity supply rather than spacecraft, the useful comparison is with what the grid already offers. Our live European grid tracker shows how much of each country’s power comes from solar and how it drops at night, which is the gap space solar is meant to fill.

References

1. G. Kopp and J. L. Lean, “A new, lower value of total solar irradiance: Evidence and climate significance”, Geophysical Research Letters 38, L01706 (2011). doi:10.1029/2010GL045777
2. PVEducation, Standard solar spectra (ASTM G173-03 AM1.5 Global, 1000 W/m²). pveducation.org
3. NOAA NESDIS, GOES eclipse schedule. nesdis.noaa.gov
4. Caltech, “In a first, Caltech’s Space Solar Power Demonstrator wirelessly transmits power in space”, 1 June 2023. caltech.edu
5. ESA Space Economy, “Space-based Solar Power: Contributing to achieving Net Zero by 2050”, August 2022. esa.int
6. NASA Office of Technology, Policy and Strategy, Space-Based Solar Power report, January 2024. nasa.gov