It doesn't take much power to shoot a satellite out of the sky. The hard part is actually hitting it.
Let's say you're aiming at a GPS satellite in geostationary orbit. According to Wikipedia (http://en.wikipedia.org/wiki/Geostationary_orbit) you're trying to hit a target that is 20,000 miles up in the sky moving at about mach 10, and the target is the size of a car. We don't have the tech for that kind of dumb-fire accuracy, even if you fire the gun from above the atmosphere. At 20,000 miles, you would need to reliably place a shot within slightly less than 1/1000th of an arcsecond.
In order to hit a target under those conditions, you would need a projectile that can do course corrections on the way to the target. Like a missile.
Let's say you're aiming at a GPS satellite in geostationary orbit. According to Wikipedia (http://en.wikipedia.org/wiki/Geostationary_orbit) you're trying to hit a target that is 20,000 miles up in the sky moving at about mach 10, and the target is the size of a car. We don't have the tech for that kind of dumb-fire accuracy, even if you fire the gun from above the atmosphere. At 20,000 miles, you would need to reliably place a shot within slightly less than 1/1000th of an arcsecond.
In order to hit a target under those conditions, you would need a projectile that can do course corrections on the way to the target. Like a missile.