What exactly is space-based solar power, and how do the competing approaches differ?
The short answer. space-based solar power means collecting solar energy in orbit and delivering it as useful power somewhere else, usually to the ground. Four families of design compete. Microwave systems in geostationary orbit, the classic concept, convert sunlight to electricity, then to microwaves beamed to large receiving antennas called rectennas. Laser systems, usually in lower orbits, beam infrared light to photovoltaic receivers. Reflector systems use mirrors to send sunlight or concentrated light to solar farms on the ground. And power-to-space systems beam energy to other satellites rather than to Earth. Each has a different efficiency, receiver size, safety case and first market. Space-based solar power is not a way to make solar cells work better: it is a long-distance wireless power system with a solar plant attached.
The four families
| Family | How energy travels | Typical orbit | Receiver | Main advantage | Main problem |
|---|---|---|---|---|---|
| Microwave (classic SBSP) | Sunlight → DC → microwaves (2.45 or 5.8 GHz) | Geostationary | Rectenna kilometres across | Passes through cloud and rain; high end-to-end efficiency | Huge transmitters and receivers; mass |
| Laser | Sunlight → DC → infrared laser | Low or medium orbit | Photovoltaic receiver metres to tens of metres across | Small receivers; small spacecraft | Clouds block; low conversion efficiency; eye safety |
| Reflector | Sunlight reflected or concentrated by mirrors | Low orbit (Reflect Orbital) or geostationary (Overview Energy, near-infrared) | Existing solar farms | No conversion in space; uses existing receivers | Brightness, astronomy, coverage time |
| Power to space | Light or microwaves to other satellites | Various | Satellite arrays | Customers already exist in orbit | Small market per customer |
Exhibit 2.1
The two main families serve different customers
Microwave from geostationary orbit versus laser from low orbit
| Microwave from GEO | Laser from LEO | |
|---|---|---|
| Spot on the ground | Kilometres | Tens of metres or less |
| Weather | Passes cloud and rain | Blocked by cloud |
| Chain efficiency (Setcoin) | ~18% | ~5.5% |
| Scale | Gigawatts | Kilowatts to megawatts |
| First customers | Utilities, governments | Defence, remote sites |
What it is not
| Misconception | Reality |
|---|---|
| "Solar cells in space are much more efficient" | Cells are the same; sunlight is slightly stronger (about 1,361 W/m² versus about 1,000 W/m² peak on the ground) and available almost all the time |
| "It replaces ground solar" | It competes with firm, round-the-clock power, not with daytime solar |
| "It is like a satellite broadband system" | It must deliver megawatts to gigawatts, not watts, which drives scale |
| "Launch is the only problem" | Hardware cost, assembly, efficiency and receiver land are equally hard |
| "A death ray" | Beam intensity at the receiver is designed below sunlight for microwaves; the safety case is engineering and regulation, not fiction (Chapter 35) |
Why the distinction matters
The four families have different first customers. Laser systems with small receivers suit military bases and remote sites that need kilowatts to megawatts. Reflector and near-infrared systems that light existing solar farms target night-time supply for large electricity users such as data centres. Microwave systems in geostationary orbit target firm grid power at gigawatt scale. Power-to-space systems sell to satellite operators who need more power or longer eclipse coverage. A reader assessing a company must first ask which family it belongs to.
The terms used in this study
| Term | Meaning |
|---|---|
| SBSP | Space-based solar power |
| SPS | Solar power satellite |
| Rectenna | Rectifying antenna that converts microwaves to electricity |
| Power beaming | Wireless transmission of power by microwaves or light |
| LCOE | Levelised cost of electricity: lifetime cost per unit of energy delivered |
| Firm power | Power available on demand around the clock |
| Specific mass | Mass in orbit per kilowatt delivered to the ground |
What would change this view
A demonstration that delivers meaningful power (kilowatts and above) from orbit to the ground would move SBSP from concept to engineering for all families. Advances in one family, for example very efficient lasers, would shift attention towards it.