Indoor Yield-O-Rama

Errors Introduced From Misrepresenting The Use Of Light

The precise degree to which light is misrepresented in watt-based ratings depends on the LPW rating for both of the lamps being compared. As these two 400w lamp examples from the previous page illustrate,

Crop Yield
Space Light
total lumens
(g/sq ft)







28.4 5.8 0.71 123





28.4 7.4 0.71 96

the MH yield should have been rated 28% higher than the HPS (LPW's 123/96=128%), and the HPS yield should have been rated 22% lower than the MH (96/123=78%). However, being blind to light and to the LPW-gap between these two lamps, watt-based ratings essentially misrepresented light by underrating the MH lamp's yield by 28% and overrating the HPS by 22%, which in this particular case resulted in identical 0.71 g/w ratings. Given the 22 different HID lamps shown in the LPW table, there are literally hundreds of different lamp combinations just like this, each with different LPW-gaps and degrees to which they will misrepresent light.

The chart below shows combinations for 13 common HID lamps when each is being paired with a 1000w HPS lamp having 135,000 lumens. The % figures indicate the disconnect occurring in watt-based ratings, and by how much the error would underrate or overrate the yield in a g/w rating.

Click for the LPW references used in the chart!

A 1000w HPS lamp was used in the chart because quite often grams/watt figures originating from smaller lamps are converted by some to appear as if they all originated from a common 1000w HPS lamp. The conversion, determined by the wattage of the lamps (1000w/100w=10), is then applied to the yield from the smaller lamp to make it appear as if it originated from a lamp with 10 times as much light. Ironically, this uninspired attempt to account for lamp differences by using an equivalent wattage still doesn't account for equivalent light, the same LPW-gap errors are still present. The resulting g/1kw rating is, unfortunately, another red herring. In the chart, the colored numbers under the bars indicate by how many watts this equivalent g/1kw rating misrepresents light. For example, using the same conversion, but with the lumens from a 100w HPS, its light would in fact be equivalent to that originating from a 680w HPS lamp (9,200*10=92,000), not from a 1kw HPS having 135,000 lumens. The 100w HPS garden's yield would still be underrated by -32% (92/135=68%) whether it's expressed as g/w or g/1kw.

An unfortunate consequence of using watts as a measure for light is that growers using smaller lamps, and who try to emulate g/w yields from other growers who use larger lamps, often fail to reach their g/w expectations by emulating what the other grower does, or how he uses light. This isn't because they're not working hard at improving their technique, but because the g/w figures they're trying to emulate often originate from a more efficient lamp, and not necessarily from a more efficient technique. A cross-lamp rating based on light, such as g/kL, would have pointed that out. Looking too hard at the g in g/w can make it easy to overlook the fact that the w, when misinterpreted to mean light, doesn't apply equally to gardens using different lamps.


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