You can probably balance things out a bit. I don't think that "30% efficiency/loss" is the best way to think about this. Instead, think of it as "extra amount

You can probably balance things out a bit. I don't think that "30% efficiency/loss" is the best way to think about this. Instead, think of it as "extra amount of sunlight reaching Earth" (and subtract the extra amount of radiation, which would probably be a second-order effect). If you've a 100m mirror illuminating the night-side of the planet, you'll need a k \* 100m size mirror shading the day-side part... And you'll probably need to calibrate this depending on the region you illuminate/shade as different parts of the Earth reflect/absorb different amounts of sunlight ("efficiency loss" I guess). All in all, a very interesting problem indeed! I can't wait until humanity can "afford" such large-scale geo-engineering.