one Gigawatt per Km squared

"A satellite capable of beaming one billion watts of solar-generated
electricity back to Earth would probably need a solar panel with an area
of one square kilometre. But spider robots could also be used to build
massive communication antennas or a shield to protect satellites from
orbiting space junk." New Scientist

It sounds like > 70% efficient. That number is not believable.

Three square kilometers, maybe. Four square kilometers , probably. The
author needed to look up insolation values and feak conversion
efficiencies and didn't do it.