Howdy,
Watts per gallon is a little misleading. The more important "measure" is what type of lighting will be the source. Let's see, 5 watts per gallon for a 180 is a total of 900 watts.
You could put 22 NO fluorescent bulbs over the tank to get that wattage. I doubt that is your plan, but you can get an idea of why watts/gallon is not used.
Anyway, lets assume your 180 is a standard 6'x2'x2' tank. I would recommend 3 400w MH, most likely using the Iwasaki reef bulb. Then add 2 6' VHO actinic bulbs to give some actinic supplementation, if you feel you need it. I have a similar setup on my 180, but I am using only 3 400w 10000K bulbs with no actinics.
Good luck,
Scott.