Darn it, stupid internet kicked me off after I wrote a whole page practically lol
Anyway PAR is parabolic aluminized reflector light rating.
So wattage is just how much electricity that the bulb is using so therefore you can have a 5 watt Incandescent bulb and a 5 watt Fluorescent bulb that have two different ratings. You probably know that Incandescent lights are very dim, they put out extremely low PAR where Fluorescent's are much brighter.
So basically it has to do with a lot about the reflector and what kind of bulb is being used. The stronger and better reflector you have (like those work lights I showed you) then the better it will reflect into the aquarium, giving better light. make sense so far?
So theoretically, if you have a strong reflector panel your incandescent could be a good light source, but it's still not strong enough and doesn't put out enough light. So I suggest getting either a Fluorescent with a good reflector or a CFL (compact fluorescent light) with a good reflector.
But anyway, it's not all about the reflector, it also has to do with Kelvin ratings. So many incandescent's are 5,000K and below on their rating while fluorescent's are usually at 6,500K. What's the big deal with K ratings? Well we've measured the sun at on a clear, mid-day afternoon at it's peak it's actually coming in at just about 6,500K so we look for lights that simulate that sun at that time of day! So that's why we look for 6,500K and higher. Higher K ratings will benefit higher light plants but it doesn't always mean you'll have a high PAR rating.
Any of that make sense? lol I have links to help explain more but they're from a different forum so just let me know if you want to see them and I can PM you