If your using regular light bulbs in the apartment switch them all out for low watt fluorescent bulbs, and another thing people don't understand is that even when appliances or electronics are turned off at night or when not being used, but still plugged in, it still sucks up energy. So there are lot's of changes you can make to make up for some of that extra power used on the grow. Change all your lights in the apartment and hook all you electronics up to one power bar and unplug it when your not using anything, that alone will save you lots.
By the way, that unplugging everything tip. I got it off the Oprah show "I know I'm gay, lol" but the one girl was showing how she split her electric bill in HALF just by unplugging anything that's not being used and apparently it works. I takes time and discipline to do it, but it adds up, and the more things you plug into one power-bar makes it easier to just unplug the one plug each time.