In a 6.5X7.5 space I did two runs with 2 Solar Storm 440s and two 600w HIDs.
In the first run, there was one plant (same strain) under each light. The two plants under the SS440s yielded about 4.5 ounces each for a total of 9. The two plants under the 600w HIDs yielded 6 and 7 ounces for a total of 13 ounces. Note that the LED plants had the benefit of some spillover light from the HIDs, while the reverse was not true. Also, the LED plants needed 10 more days to finish.
The second run was all the same, except that the LEDs did perform a little better. I got a total of 10.5 ounces from the two plants, while the HID plants came in about the same at 13.5 ounces. Again, the LED plants had the benefit of spillover HID light and again they needed more time to finish (8 days).
I AM NOT ARGUING AGAINST THE FACT THAT LED IS MORE EFFICIENT, WATT PER WATT. I AGREE. Math proves that...
... except for when time is added to the equation, at least in my limited experience. Meaning... the HID plants produced roughly 30% more in slightly less time.
Regardless of what math is used to prove efficiency, the 30% more yield in the same space and roughly the same time is a HUGE factor.
This experiment of mine, while not scientific, matches what I have heard from others who grow around here with LED.
So I go back to my original question, which is not about efficiency... is it possible that LED will at some point be able to produce at least equal to HID in the same grow time?