It can't be that easy...

[Just5398]

is figuring out the wpg as easy as just dividing the bulb wattage by the tank size?

[Sploke]

thats pretty much how it works. the decisions you make based on that decision are where it gets harder.

[Quartermain]

That's where it starts. Then you take into consideration bulb-type/temperature, tank depth and plant need.

[jmhart]

For tanks 20-75 gallons it's pretty much that easy(factoring in K value, as mentioned). If it's less than 20g or greater than 75g, the rule starts to deviate. Use 6500K bulbs or above.

[echoofformless]

WPG really isn't as important as they say it is.

Things like the type of lighting (PC, NO, HO, T5 etc) the color temperature, the tank depth, and the amount of lumen produced are far more important than wattage. Put it in your mind that wattage doesn't necessarily translate to actual light produced - just the amount of energy that the bulbs use. Think of say, a small but high tech auto engine with four valves per cylinder and turbo charging vs. a large but low tech engine with only two valves per cylinder. The smaller engine will produce more horsepower with less energy consumption. Logic says go with the smaller high tech model.

For example I was running four NO fluorescent T12 bulbs over my 55g with a total wattage of 160. (2.9 WPG.) Growth was okay, but I also got lots of brown algae.

I moved to a 2x65w (2.3 WPG) PC fixture and not only did the brown algae disappear, but the growth on my plants exploded. It all comes down to the higher lumen output and the lowered restrike of the PC bulbs vs. the T12 NO fluorescents. Then I added a T5 fixture (an additional 56w) and my plants are making a small jungle.

So look more into the type of lighting. T5 and PC are a lot brighter at the same wattage than larger diameter fluorescent bulbs. High output T5 have been very well received and some are considering them a new standard in aquarium lighting. If money isn't such an object, I would look into them.

[phanmc]

Color temp (K value) for the most part is unimportant and so is lumens, which is a measurement weighted for the human eye and not necessarily what's best for plants.

Light type is important. T5s and PC lightings are better than regular T8-T12 fluorescents. If you use good lighting stick to the lower range, if using regular fluorescents stick to the higher range.

The wpg rule works well as a general guideline IF your tank is in the 20-75ish range like jmhart mentioned. If you have unusually shaped tanks or very tall/short tanks you'll need to play around with it a little bit and do some guesswork.