What I remember of this novel/series is that the main character creates solar laser satellites and mines asteroids to create more satellites to defend Earth from aliens. Later on, he uses the hollowed out asteroids as a base and eventually as an armored fortress to repulse the alien fleets. I last remember the main character attempting to hollow out more asteroids to defend and attack the aliens eventually.
This is somewhat sparse on details however it sounds like the Troy Rising series written by John Ringo
The protagonist puts mirrors in orbit to focus the sun and he does create a very large battle station out of an asteroid.
Troy Rising is a book in three parts—Live Free or Die being the first part—detailing the freeing of Earth from alien conquerors, the first steps into space using off-world technologies and the creation of Troy, a thousand trillion ton battlestation designed to secure the Solar System.
Citadel continues the saga begun in Live Free or Die, following the paths of several characters during the first years of The Spiral Arm Wars culminating in the First Battle of E Eridani.