600w will use 5.5amp (600w light + 10%(60w) ballast=660w)
660w / 120v = 5.5amp
a typical electric space heater runs at max 1500w, or 12.5A draw. your electric company is not going to notice anything suspicious from a single light operation, no matter when you pay your bill.
the most common ways to get busted are odor and, primarily, lax security. someone opens the wrong door looking for the bathroom. your buddies are over and you want to impress them with your operation. someone tells someone else who doesn't like you as much and they happen to need some leverage with the police. next thing you know you're face down wondering wtf happened, and this guy is laughing his ass off because he just got away with something at your expense. bottom line, keeping the whole deal under wraps is more important than your power consumption.