Re: @Destroy All Monsters
The Moon doesn't shine, it only reflects (very badly, too), but still:
1.3 kW/m² dumped on the moon by the sun.
Make it 2001-monolith black, all surface utilized...
1737km radius gives you are disk of a bit less than 10^13 m² to shine on.
So you can collect 1.3 x 10^16 W, which is nice.
According to E = m*c², this corresponds to 0.14 kg/s in case of perfect backconversion.
So you are really looking at a few billion of years full-out collection.