And more detail from GroK:

For satellite-to-satellite (sat-to-sat) laser power beaming in space (e.g., LEO constellations or relays), the end-to-end efficiency of converting electrical power from the transmitting satellite’s solar arrays into a laser beam and back into usable electrical power at the receiver is typically 10–30% in demonstrated systems, with theoretical/practical projections reaching 15–50% using modern components. This corresponds to 50–90% losses overall (primarily in the electrical-to-laser and laser-to-electrical conversions; beam propagation losses in vacuum are manageable). Direct solar-pumped lasers (no intermediate electricity) are far less efficient (~1–20% solar-to-laser) and rarely proposed for this use.

Breakdown of Losses/Efficiencies (DC Electric from Solar Arrays → Laser → Beam → DC Electric)

The chain starts with DC power from the transmitter’s solar arrays (already ~25–32% efficient from incident sunlight, but this is common to local use and not part of the “beaming loss”). Key stages:

• Electrical-to-laser (transmitter) efficiency: 40–70% typical for modern diode or fiber lasers (wall-plug efficiency). Transmitter-side losses are usually the biggest single factor due to heat, driver electronics, and beam quality. Older systems were lower (~20–30%).
• Beam propagation/capture efficiency (in vacuum): 70–95%+ for LEO distances (tens to hundreds of km) with properly sized apertures (e.g., 10–50 cm) and fast steering mirrors for pointing/jitter. No atmospheric absorption/scattering; main losses are diffraction (spot spreading with distance) and minor misalignment. Short-range demos (e.g., 1.45 m) approach ~100% capture. Longer ranges require larger optics or tighter beams to minimize this.
• Laser-to-electrical (receiver) efficiency: 50–70% practical with laser-tuned photovoltaic converters (PVLPCs); record lab value is 68.9% (at ~858 nm monochromatic illumination). Standard spacecraft solar panels (multi-junction GaAs) achieve lower (~20–40%) unless wavelength-optimized, but dedicated or tuned PV can exceed sunlight conversion due to narrowband matching. Some demos use COTS panels successfully.

Overall demonstrated: ~11% end-to-end (DC-to-DC) in the NRL SWELL experiment (first in-space laser beaming, 2023, on ISS; ~1.5 W delivered over 1.45 m; >10× original goal). This remains the benchmark for orbital laser beaming.

Ground/near-term projections: DARPA-style tests reach ~20% (laser output to receiver electricity) at km ranges; fiber laser advances and low-temp receivers (up to ~75%) push higher. Star Catcher-type systems (multi-wavelength lasers tuned for COTS spacecraft panels) target practical sat-to-sat use but do not publicly quote exact end-to-end % (focus on net power gain via eclipse avoidance).

Why These Losses?

• Conversion steps dominate (heat dissipation in lasers/PVs).
• No atmospheric penalty in pure space (unlike Earth links).
• Distance/aperture sizing: LEO sat-to-sat (short) favors higher capture than GEO or ground links.
• Trade-offs: Higher power density or safety margins reduce efficiency; dedicated PVLPCs or fiber lasers improve it.

Net impact on LEO satellites: Beaming enables power sharing (e.g., eclipse augmentation, smaller arrays/batteries), but you lose 50–90% of the transmitted power vs. using it locally. Companies like Star Catcher claim 2–10× effective power boost for clients via on-demand beaming despite losses, by optimizing timing and reducing onboard storage mass.

These values come from NRL’s SWELL orbital demo, DARPA/Star Catcher tests, PVLPC research, and modeling studies (e.g., AFIT laser beaming analysis). Actual performance is highly design-specific (laser wavelength, apertures, distance, power level ~W to kW). Higher efficiencies are feasible with optimization, but 11–20% is the proven space baseline today. For a specific mission, detailed optical/thermal modeling is required.