By Carl Strohmeyer
When choosing your aquarium lighting (especially Reef or Planted Freshwater), there is much more to consider than watts per gallon.
In fact is the 2-5 watts per gallon 'rule' is a VERY basic start (this rule varies depending upon what is kept & who you ask), however this is very general and quite out-dated due to the variety available modern lights with varying lumens per watt lights, different wavelength, focused lumens, PAR/PUR output, etc.
Sadly the watts per gallon "rule" for aquarium lighting is still thrown around today despite all the technological advances in lighting which often makes this grossly inaccurate.
As an example, a high PAR/PUR LED from the most exclusive/best emitter bins can only need 4% of the wattage as a household T12 to produce the same results (about 15% for better aquarium T8 or T12 lamps), but even then many LEDs available such as those sold by mass merchandisers are not of high useful PAR output (PUR), as these often use much lower quality emitters with poor PUR output.
Article continued at link below