Must have a lot to do with the lighting for reef systems(i.e.MH). I have my tank in the basement. Keep in mind I just upgraded to a 180 from a 45 so I am not sure what the impact will be on it now. Hopefully not a huge amount.
I compared all 2003 (didn't have the tank) and 2004 (did have the tank) I only have 110 watt pc and 75 watt NO. I didn't see a difference at all. some months it was more some months it was less. So as far as energy cost goes I only have about 20 dollars more this year so far.
I had a few months in the summer when my bill was higher but the air was on more this year then last seemed to be a bit more humid. So I attribute the higher cost to that not the tank seing that every other month was relatively the same.
I am not complaining a bit.
Oops knowone ever told me I had to count everything.
In that case its probably about 50 to 100 a month.