Other factors besides video card usage. Other computer components, fans, and air conditioning. If you air condition your home any energy that goes into the home is just as costly to extract from the home. In the winter it may make more sense but if you are using 200 watts as your metric, you are way off.
I was just trying to keep it simple. I have a rig of 4 5830's that pull 900 watts from the wall that's 225 per card (if you average in the watts used by mobo and cpu ect.) If you add a 5th or 6th card to the rig it goes very close to 200 watts each. So that part of what I was saying is correct.
When you say you need air conditioning that's not entirely true. You can throw your rigs in a basement and they will be perfectly fine with out ac and you wont be effected that much by the heat. If a basement is not an option then I'll point out that opening the window now is its own form of ac for the northern states. It gets to between 50-60 degrees at night which is as low as any ac will go. During the day it doesn't really get hotter then 80 degrees so you could just suck it up and not use ac because the computers wont be harmed it will just be a discomfort for your self. This discomfort will slowly go away though as it gets colder.
Any other factors I should take into account?
I never said "you need air conditioning", I said "if you air condition your home". Putting it in the basement is irrelevant unless your basement is separately insulated as hot air rises and will end up upstairs otherwise. It is a simple energy in, energy out equation. If you put 900 watts in, in the form of waste heat your AC has to pull those 900 watts out.
With winter approaching this is not much of an issue. In the winter months that extra waste heat is good as it reduces your heat bill. I also live in a northern climate and this week has been < 70 degrees, but last week topped out at 95. This year is probably the first that I am looking forward to winter.
