In deciding how much light your tank needs, don't think about watts per gallon. Instead think about how far from the light the substrate is. If your tank is 48" x 18" X 22", the light will be about 22 inches from the substrate. At that distance a T5HO bulb, with a single bulb reflector made of highly polished aluminum, will give you low medium light. Two bulbs close together will give you high light. Any more than that will give more light that you are likely to be able to manage. A pair of T5NO bulbs, with the slightly less effective reflector typical for them, will probably give you about the same light as the single T5HO bulb. Your tank is 18 inches front to back depth. That makes it difficult for a single light to evenly light the whole substrate, and plants would all grow towards the light if you did use it. So, it is better to use two T5HO bulbs, widely separated, or two 2 bulb T5NO lights widely separated, to light that tank. Or, many of us like to suspend our light a foot or so above the top of the tank. This makes maintenance easier, gives more uniform lighting throughout the tank, and makes adjusting the light intensity as easy as raising or lowering the lights a little bit. If you do that, you could use a 4 or 6 bulb T5HO light. You will use more electric power than necessary that way.