I was looking around for the math to find the cost to run one 400 watt hid light.This what I got. WATTS X HOURS = TOTAL WATTS PER DAY.12HOURS X 400WATTS =4800 WATTS PER DAY.DIVIDE TOTAL WATTS PER DAY BY 1000 TO GET TOTAL KILLOWATTHOURS PER DAY.4800 / 1000=4.8 KILLOWATT HOURS PER DAY.MULTIPLY THE KILLOWATTS DAYS TIMES THE COST PER KILLOWATT TO GET THE TOTAL COST PER DAY..11 X 4.8 = 0.528.COST PER DAY TIMES 30 DAYS = COST PER MONTH. 0.528 X 30 =$15.84 . This is with in a few cents of my bill each month doing the math this way.Peace