Thank you, I was going to make the same point. Another point worth considering is that if the biology of the aliens were Earthlike, it would be nice to have th

Thank you, I was going to make the same point. Another point worth considering is that if the biology of the aliens were Earthlike, it would be nice to have the "native" intensity of sunlight (or close to it) available everywhere on the inside of the sphere. Even if the biology were oriented to 4x less insolation, that would only be a 2x increase in sphere radius. I think the "efficiency" concern is a bit silly also, since advanced alien civilizations will have advanced, high-density power sources available as needed. The only rationale I can see for very high efficiencies would be the desire to hide...and that might be a valid concern.