watts X hours X your KW cost / 1000
400 12 .08 1000 = .384 thats 38 cents for 12 hours
400 12 .17 1000 = .81 thats 81 cents for 12 hours
The formula is right and it works you can find KWH cost on your ele bill. But at the end of the month it won't add up. The reason being is that other number called multiplier. only the ele company knows what it's going to be and it changes every month if the power company wants too. That is there way of screwing you. The only way to tell for sure is take the KWH they charged you for and then divided that number into total cost on the bill

billboards are free often 1000 hps/mh well just dont get caught....lol...just takes 1 to get started!