From The Aquarium Wiki
WPG stands for "Watts Per Gallon".
This refers to a rough guide or 'rule of thumb' on the amount of light (typically using T8 fluorescent tubes) that a aquatic plant needs to grow.
So if you have a 8 Gallon tank and you've got a 20W tube over it, then it is said you've got '2.5WPG' of light (20 divided 8).
The average aquarium plant will require 2-2.5wpg to grow successfully.
Obviously this guide is very rough as it does not take into account the depth of the tank.
- The Gallon in this case is always in US gallons, so UK aquarists will need to convert their UK gallon to US Gallon (multiply by 1.2).