Watts Per Gallon. The wattage put out by the aquarium lights per gallon of water in the tank. For example, a 10 with two 15 watt normal output Fluorescent lights will give you 3 watts per gallon. Found on http://www.aquahobby.com/articles/e_glossary.php
1) Workstation products group 
Watts Per Gallon. The wattage put out by the aquarium lights per gallon of water in the tank. For example, a 10 with two 15 watt normal output Fluorescent lights will give you 3 watts per gallon.