Watts can be a rough indicator of lumens based on most LEDs (and actually most other light sources i.e tubes metal halide)Thanks oreo, so that would equate to about 8 lumens per litre, well below the 10-20 level that's often quoted for low level plants.
Having slept on it @oreo57 it begs the questions that lumens in and of themselves are meaningless in relation to plant growth, which isn't really ground breaking news.
Or these modern led's put out far more par values than the wattage would once of a day suggest there capable of.
The light compensation point of aquatic plants is far below what was previously assumed.
Or I've somehow got lucky and managed to defy the laws of physics with my lights.
They have an efficiency of about 70-100L/watt.
Old style 3 watt-ers were around 40-60 though.
Yes lumens are err "antiquated" in this day and age. Does help to at least compare led lights.
Mostly because the "delivery" is equal.
120 degree beam angle for 80% of the lights out there.
No worries of reflector differences and not much for shields except if "frosted".
Next measurement up would be LUX which is lumens/area.
Using simple math and some stats one can determine LUX
Lumens (lm) to lux (lx) conversion calculator and how to calculate.
Then LUX to PAR.
Online calculator to convert illuminance (lux) to PPFD (micromoles per second per meter squared).
But complicated and prone to errors though it can get close.
"PAR" or more correctly PPFD is the defacto standard.
So since LEDs are very directional and usually hung low on the tank you can "beat" physics by not wasting photons.
At one time standard thinking was 1/2W LED = 1W t5
Efficiency was similar, delivery was not.
LED's currently are in the 120-150L/watt 20-50% more efficient than tubes.
That's diode efficiency not system but if using "state of the art" diodes and the delivery efficieny you can "beat"
T5's ect. easily in less watts/PAR
Last thing is lumens are weighed around the green spectrum so blue "lumens" are recorded as less than green
lumens though they (assumption) both generate the same amout of photons (PAR).