You would use 288,000 kw every 30 days if you ran a 400 watt bulb 24/7.
The math is 24(hours a day) x 400(watts) = 9,600 kw a day x 30 days = 288,000
Multiply 288 x .10 (your electric cost estimate) and get $28.80. This would be your cost per month if you used your bulb 24/7.
Now you can figure out you total usage by the hour. Divide 28.8 by 24 and you get $1.20 an hour. So 12 hours would be $14.40 per month. 18 hours would be $21.60 etc