WPG is outdated now-a-days, PAR data and lumens is a better way to figure your lighting. Odyssea 48" T5HO quad systems are around 5,000 lumens per bulb which is why most people will tell you that a single T5HO bulb is enough lighting for a 55 gallon tank.
Now with that said I prefer using Quad T5HO setups for my tank because it lets me run 2 x 6500k bulbs from 9am to 12noon, and then use 2 x 5000k bulbs for afternoon from 12noon to 4pm. Because 5000k bulbs reflect late afternoon sunlight better than 6500k bulb do.
45g Long - High Light Planted Tank / 30g - Plant Grow Out Tank / 100g Tank - currently resealing