its just something some marketing dude came up with.
basically each type of tube is given a name - like plant pro, powerglo, bio-lux, flora pro, aqua glo etc etc.
power glos have to be really really bright. and i mean unusually bright for a tube. this where the power part of its name comes from. it also is an 18000K tube.
life glos for example are 6700K
this is where it gets confusing.
aqua glos are also 18000K - just like the power glos - but are not as bright, because they are not the "powerful" ones - this is how i remember it.
The wattage of the tube is the same across the same size range - but the amount of light (in intensity) varies.
you can get a dull 36W tube, and also get a really bright 36 W tube; thus my argument against the whole WPG calculation.
eg- I have t5 High Outputs (54W x2) which use a total of 108W of power - but definitely emmit more light than 108W of "normal" light. thus the $200+ price difference.
In the same way a 36W power glo is brighter than a 36W aquaglo.
You can also look at the tubes and compare them if you don't believe me :lol: