Thanks everyone for your responses.
I was surprised by the $40 additional cost MoonFish said a new tank added to his monthly bill. With my 40 gallon, I purposely stuck with 4 bulbs NO to keep the costs down. That's not really possible with a 75 gallon due to the width. Plus, reading Shalu's sticky thread, it seems that NO is, in any case, also inefficient as it under drives the bulbs. Has anyone ever looked at what is the most efficient ODNO load for a 32 watt 48" bulb? eg, 2X.
Looking at the first response to my question, the formula for computing actual wattage: (4x 32 watts x.88) x .75 heat loss) x 3 ballasts = 253 watts = 3.4 wpg on a 75 gallon tank. If I want to get 200 watts (2.7 wpg) or thereabouts, what ODNO scheme would you recommend and would there be substantially less cost per month? I'm just looking for reasonable high light at a reasonable monthly price. I'm now running 2.7 wpg on my 40 gallon and it works great for me.