Yo dude, LED growlights usually use (or draw) less wattage then they throw out as 'light output' wattage. 3 Watt LEDs arnt actually powered at 3 Watts, the sweet spot is somewhere around the 2.22 Watt mark, if you powered a LED any higher then that you don't get any more 'output' or brightness just less effiency and you shorten the lifespan of the LED itself. Even tho the LED is powered at a lower rate the light output is meant to be equivalent to 3 Watts equivalent light output value. Ie PAR/Lumen measurements. So some suppliers rate their LEDs by the output, some rate them by their power usage.
Lumens is a wonderful measurement for what is was created for - measuring light as it affects the human eye. Since we see better in blue than in red, you have higher lumens with just a blue light...which as we know wouldn't grow very good bud. Umols is the unit of measure that you are looking for to judge light effectiveness for growth; it measures photons in a waveband....say the PAR band...