I have actually tried putting 400 watts of lighting over a 10 gallon tank of regular light bulbs and got plants that melted away and turned into mush. A small bank of 4 light sockets with 4, 100 watt regular light bulbs with cone reflectors and actually had a huge amount of light, no growth but the tank was very bright.
I actually have well over 3 watts per gallon on my 10 gallon tanks right now and I can not grow the higher light plants and have rarely seen any reds that you get from actual high light.
You must have enough watts, proper lumens, and proper spectrum to grow plants. 6500 K or higher, a CRI as high as possible and the highest lumens possible. If it was not for this FACT then I could put 30 watts of standard incandescent screw in household bulbs over my tanks and grow plants as well as I could with 30 watts of power compact flourescents with the proper K, CRI and Lumens. If all you need is wattage then why not get nothing but actinic lights for fresh water plants? Because they will not grow plants!
Yes you have to have enough wattage but without the proper Kelvin rating, Color Rendering index and Lumens the light is useless.
The w/g "RULE" no longer stands alone as the standard for lighting. http://woo.gotdns.com/Aquarium/Lighting.htm http://www.rexgrigg.com/mlt.html
More and more aquarists are using the lumens/watt/square inch or LUX standards including Tacheshi Amano, Rex Grigg and Travis Simonson The watts per gallon rule simply does not work anymore all by itself. Example, for very high light you need 7.5+ w/g for a 10 gallon, for a 55, you need 4 w/g to achieve the same high light standard.
I hope this hasn't confused you teh13371, we just want to make sure you have the best chance of success at growing plants and get all the FACTS to do it right. Please ask as many quesitons as you need to to make sure you understand the whole thing, or at least as much as you need to.