no one is checking everyones power bill to see jumps in the bill
if they do check it its because your already being investigated and that means that already know alot more than what your powerbill will tell them
i use a 250 and on 12 12 its like 10 bucks on 18 6 its like 15
400 would presumably almost double that
and thats as high as you want to get for just a small closet grow (which is what i presume your doing being in an apartment and all)