Lighting is very impotent we all know that but the write spectrum is very important sunlight spectrum for plant growth is 6000k(kelvin) to 6500 this is the range of most planted aquarium tubes but intensity is also important this is ranged from 0 to 10 for planted tanks any thing above 8 is good
so here is the notation for lighting which is available in most electronics stores just ask for T5 bulbs which is numbered 865 or 965 or 860 or 960 first number that is '8' stands for intensity and the last two for temperature 65 = 6500 or 60=6000 believe me u r going to save lot of money by knowing this instead of wasting your money on branded aquarium tubes which are low in quality coming to the wattage i use two 14watts T5 tubes for my 55 gallon tank which is enough for me but u can use 3 for the same tank ...hope i helped u :)