Watts per gallon is used to define levels of lighting largely because all light bulbs have their wattage printed on them somewhere, or at least on the packaging. Lumens are only occasionally printed on the packaging. And, no other measure of the brightness of the bulb is easily available to us. Since lighting available to the plants is not something that can be precisely measured in any case we just use the easy to determine watts per gallon, knowing it is only an approximation. In today's usage, when someone specifies that a planted aquarium needs, say, 2 watts per gallon, they mean 2 watts per gallon of bulbs comparable to compact fluorescent tubes, that are thin, and used with good reflectors, so that more than half of the light from the bulb is directed into the tank. So, for your 30 gallon tank, for plants needing a moderate light intensity you would want about 2 watts per gallon - say a single 65 watt fluorescent T5 tube, either straight or bent into a two parallel tube configuration. To be able to grow just about any plant, you would want up to double that. I think that is about as accurate as the watts per gallon rule gets. And, finding the fixture or hood that uses that size bulb or bulbs, and has a good reflector is the next problem.