I have a grow room setup for 5- 7 plants or so and since this is my first indoor crop im looking for a light that would be suitable for a 3x3 area in a 1 bedroom apartment. I was thinking about getting a 400 watt MH light kit. How much would a 400 watt MH run up my hydro bill each bill.
If you know what lighting regime you are using, and how much you are paying for a kW-hr of electric, it is surely easy enough to calculate.
Here's a power consumption calculator that might be of some help. You need to know the cost of your kw hours: http://www.advancednutrients.com/conversion_tools/powercost_tool/powercost.html You really won't have to worry about the cost, it'll be no different than running a small space heater.