Dunno where u live but I'll give you a example. Im in canada ontario. Its 5.6 cents per kwh , so $0.056. With a monthly allotment of 1000kwh.
This is the equation ....
With a 400W HID kit, its around 456 total wattage for the setup.(bulb/ballast consumption)
456 / 1000= .456 X $.056 = .025536 X 18hrs/day = .4597 X 30 days= $13.8
So around $13.8 per month to run it for 18 hours a day, every day. Which isn't bad.
Now for the 1000kwh a month allotment , this simply means, if you go over 1000kwh , the price per kwh goes up. Around here it goes from $0.056 to $0.065 , which is barely anything and would probably only be around a couple bux extra, literally.. like 2 dollars... My bill would go from $13.8 to $16 if I was running only the 400Watter and for some weird reason went over 1000 kwh.. know what I mean?
Also... for 1000kwh... I could run 4 , 400W HID kits simultaniously for 18 hours a day, every day, for a month , and it would still only be around 980kwh , keeping me just below that. So basically what I'm trying to say is , you really don't have anything to worry about with just 1 kit.. the people who get caught are the ones with like 10 , 1000W kits. If you're still worrying, just buy some cfls and replace them with all ur incandescent bubls in ur house, shut the lights off everytime u leave a room , shut down your computer if you're not using it, etc. U'll be ok.