Why do people use watts?

It's not that easy though. Different HPS bulbs can also have quite a different efficacy. So what is the manufacturer of the fixture going to say?

And it doesn't end at bulb efficacy. A double ended HPS bulb might be rated to produce 2100umol/s @ 1000W. Is that enough data? But then the reflector absorbs 18% of that light and only say 1750umol/s comes out of the fixture.

Now you hang this light in a 4'x4' tent. The walls are really close and the fixture needs to hang quite high. You lose a lot of light on the walls. Measurements show only 1380umols/s is then left and the walls absorbed yet another 21%. If you hang this same light in a massive green house the walls are so far away from most lights that wall losses are inconsequential. So the full 1750umol/s would apply for all but the outer ring of lights.

What umol number out of all of these should they use? It's a fixture that can run a 1000W HPS bulb though. So that's what goes on the box.

So you could argue to just report the 1750umol/s figure, but In actuality you could also alter the reflector. Some reflectors will lose 30% of the light. So the fixture would then only produce 1470umol/s. When the reflector gets dirty it quickly absorbs more light too.

The permutations pretty much never end.

With leds you normally do not see reflectors applied, but the wall losses are still there. A fixture that hangs twice as high will end up lighting twice the amount of wall. Which means you will also lose about twice the amount of light on the walls.

So it's not just the efficacy that you need to keep in mind.

The hanging height is determined solely by the light distribution you wish to achieve. Think of shining a flashlight straight down on the floor. If it's close to the ground you will see a small circle. When you lift the light up the circle gets bigger. An led fixture is basically a lot of tiny flashlights shining down and you must lift them high enough so their light overlaps and shows a uniform light intensity.

So if you compare 2 lights the hanging height becomes an important factor too since it multiplies the wall losses.

The hanging height with 120 beam angle led should be at least the same as the radius of the circle you want it to light up (with that 120 beam angle).

So if you have 1 light over 1x1 then the maximum distance would be from the center to the corner making the distance which would be the square root of (6"^2 + 6"^) = 8.5". For a single COB the minimum hanging height over a 1x1 area would be about 8" (taking into account the size of the COB).

If you hang a led strips that go from wall to wall, the maximum distance would be the heart to heart distance from strip to strip. So say you put a led strip over a 1 foot wide area, you'd only have to hang it at 6" height to cover at the same uniformity as that COB at 8"

Now if you hang a QB you generally hang one over a 2x2 area. The bit covered with leds is about 10.5"x5.5" which means that in the corners you need to cover a bit that's (24-10.5)/2=6.75" by (24-5.5)/2=9.25. Pythagoras tells us that this distance to the corners is then sqrt(6.75^2 + 9.25"^2)=11.5". And then probably a bit higher since the clump of light in the middle will give too much of a hotspot otherwise. Say 12" at least, but probably more.

So 2 led strips covering a 2x2 tent would need to be at 6" minimum height
4 COBs covering the same 2x2 tent at 8"
1 QB also covering 2x would need to be at 12".

Resulting in double the wall losses for the QB compared to 2 led strips. That's why the professional lights coming out now are all ledstrip (led bar) based. Growcraft X6, Zeus 600WPro, Gavita 1700e, Fluence Spydr etc etc. These are massive 40"x40" fixtures spreading out the light by spreading the light point uniformly over the space. Rather than clumping all the light in the middle of the tent and then let a higher hanging height spread the light.



Same goes with full fixtures. A 40"x40" 650W fixture (or 1700umol/s output) will spread it's light much more uniformly than a 20"x20" fixture with the same output.

If you want to build something yourself look into Samsung or Bridgelux led strips. They come in 2 and 4 foot lengths. Attach them to an aluminium U extrusion as heat sink.

Great answer! Thank you!

Seems then that I can take a theoretical footprint and find my wattage and gear requirements from that.

I've been eyeing Samsung strips for a while now. They fit in my tent (if I end up there for another grow or two), and there are enough builds that I can find one to mimic.

re: clump of light in the middle - I think that's the most aggravating part of my mismatched lights - hot & cold spots all over the canopy...
I brought up using watts as a reference of power because of my 3 "300w" burples:

One is nowhere near 300w - only has (2) 100w power supplies onboard...
The other two are 197 & 215 at the wall (each with two 150w power supplies), but they're alibaba, so I shouldn't be surprised that the 197w is noticeably brighter than the 215w.

printer said:

For some reason I seem to feel light hitting a surface was in watts. I found this,








Wonder if you can then rate the leds, watts out (light) / watts in (electrical)

Led me to this.

https://www.researchgate.net/profile/Mariana_Reis8/publication/339896036_Conversion_factors_and_general_equations_applied_in_agricultural_and_forest_meteorology/links/5e6b7bd992851c6ba7fdedd3/Conversion-factors-and-general-equations-applied-in-agricultural-and-forest-meteorology.pdf?origin=publication_detail

Not that useful for us but I just needed to know.

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Seen irradiance and radiance figures where Watts is part of the figure given for units of measurement. With an accurate SPD should be able to derive all the different units used. Be it based on micromoles or watts. Tools like Alesh's spreadsheet, etc.

You could see where watts of energy are lost, or not going directly to making light. So you could derive efficiency.

Lockedin said:

Consumption is a big part of the equation when designing for sure - that and heat are why I have 3 LED in my tent instead of 1 HID!

I worked the studios most of my career, lighting guys always spoke in lux / lumens (what cameras see), and then discussed the watts needed.

It seems like a lot of discussions get lost in watts - how much power is required by a certain fixture - when trying to figure out coverage / ppfd

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Lot of discussions revolve around watts. Very useful metric, for giving general guidelines on coverage and diy design of lamps. Studio lighting guys probably own very expensive lux/light meters, but like you said they would devolve to using watts as a metric.

L.E.D guys did this to themselves. for years they tried to make claims that 300 watt led could compare to 1000 watt hid and led grew magical bud that was superior to hid. everything has been compared to 1000 watt hid because 1000 watt de hid grows/grew the best weed.

Well it made sense to compare to HPS too. People had a 600W HPS or 1000W HPS and wondered what led would replace it. Without needing all kinds of calculations to do the comparison. Unfortunately the led manufacturers lied about their figures. So there is (rightfully) a lot of bad blood on those comparisons.

A trick commonly used was to confuse people with PPFD (spot values) taken in the brightest spot under the led light and compare these to the PPF (total light output) of the HPS. Like comparing lumen to lux. When lux is a value easily manipulated by placing the sensor closer to the light. Especially when testing both in large rooms with no reflective walls (where leds with their tighter beam angle are helped also)

You actually still see manufacturers abuse these PPFD matrixes without reflective walls to propose their light is better than others. Just because their light bundles more in the centre and therefore loses less light outside the measuring area. Youtube has a lot of channels measuring lights inside reflective tents though. Like Migro and Ledgardner. In those test you see that a 2.8umol/J sphere test result can result into 2.4umol/J real life values (15% loss). Or a 2.5umol/J quantum board delivering 1.9umol/J in the same real live test (24% loss)

Either way if you're looking for a ballpark figure to compare to HPS, current leds at around 2.8 umol/J efficacy will replace a 1000W DE HPS at 625W. But then you have the boost function on most HPS drivers and then that 1000W HPS can be run at 1200W. So for that you'd need something like 750W to replace that HPS instead of 625W for the base figure.

So again, it's difficult to compare with one simple number.

Better just work out from what you want (ie 800umol/s/m2 or more or less) and calculate how to produce that from the LEDs and forget about how much HPS that would need.

Lockedin said:

Like all of us, I'm always researching new equipment to make my grows better.
But bad information makes decision making difficult at best.

Why do we use watts INCORRECTLY to compare lights?
Watts are a measure of throughput to power a device (for us a light) to a certain output.
WATTS = how fast the meter spins and how big your elec. bill wil be.
PPFD or PAR seem like they would be much better indicators.

Here's an analog argument:

2 cars - each has an OUTPUT of 300hp (horsepower for non-car ppl)
Car 1 is a 1969 Chevelle with a 350 cubic inch gas V-8 motor
Car 2 is a 2007 Lotus Elise with a 1.8 litre supercharged I-4 engine.

Again ------------------------------------ Both cars output --- 300 hp each. BUT:
Car 1 has a 1 gallon THROUGHPUT of fuel every 12 miles --- 12 mpg
Car 2 has a 1 gallon THROUGHPUT of fuel every 31 miles --- 31 mpg

It makes no sense to compare the power output of a car by the mileage does it?
So why do people compare light power by how much it costs to run?

IMHO - watts at the wall - or the fixture, are a really poor indicator of power levels reaching the canopy.

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We know roughly how many true watts we need per square foot with top end LED and with low end LEDs and with HPS so its easy to talk in watts.

As far as silly lights called 1200w or 3000w , only shitty LED companies do this and its to mislead the unsuspecting customer who doesn't know any better.

Its a great reason to not buy shitty cheap LEDs.

When the whole fixture is based on selling you a lie then how can you take any notice of the par maps they provide..

Good LED companies are fairly transparent , provide info on the diodes used, total light output, spectrum and umol/j.

PJ Diaz said:

For one thing it's much easier for any common grower to measure wattage consumption using inexpensive tools than it is to measure PAR or ppfd, which requires expensive equipment. By this point, enough research has been done to know the general range of wattage needed for HID or LED lighting per sq foot to know approx how many watts you need to adequately fill a space. Another factor is that you really can't trust the ppfd numbers put out by many manufactures. A great example of this was shown just last night on the GML show, where they tested a Chill LED board themselves only to discover that Chill was putting out false claims (they apparently only tested one strip but not the fixture, and didn't account for driver loss). For $15 you can get a tool that will tell you real wattage, whereas you will need to spend thousands on a good test of light output.

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Chill have not done themselves any favours, by failing to provide test reports for complete assembled configurations. Funny thing is that all the single bar reports at different currents, on chills page all appear to have been done at same lab that HLG uses. Kind of opening themselves up to criticism, not providing reports for complete configurations.

Interesting how the GML discussion panel was getting lost in the semantics of the word "fixture". Guys, the word "fixture", in this case means the device under test. It can be a single pcb strip/pcb board or a complete light. Temp measurement would be where ever they fixed the sensor onto product.
Talk on penetration and low hung light vs high hung light seemed to leave people with more questions than answers.

Efficiency=reduced wattage draw, reduced HVAC cost, ROI..... Tech would have been dead in the water with no incentive to switch like halide LEP

What metric would you like to use? Liters of water used? Lol

I miss incandescent bulbs, they used to ping and ring.

Growing populations and energy concerns have made us so aware of energy consumption. The message is to strive to squeeze out every last bit of efficiency.

end_of_the_tunnel said:

Growing populations and energy concerns have made us so aware of energy consumption. The message is to strive to squeeze out every last bit of efficiency.

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And I'm cheap.

Agreed to all - and as I expand, cost of operation is certainly a concern - more lights, more heat, more fans, more $$$...which I'd like to spend somewhat wisely.

Cost concerns were the main reason I chose to grow in a tent under LED at first instead of converting and lighting an entire room. Glad I did wait - I would have picked up a lot of wrong equipment and become really frustrated...

Watts at the wall represents money leaving my wallet - so yeah, a VERY important number.
I wasn't trying to minimize the importance of cost - just whining that I'd rather look at efficacy, then figure cost.

Maybe I think backwards, but I want to know what the light does at the canopy first.
Knowing what a light will deliver at the canopy (X distance) will tell determine many of a given fixture are needed for strong, even lighting across the canopy footprint.

*****VERY enlightening discussion (no pun unintended!)! You guys have made my learning curve much steeper!

I have not found a test for panels but here is some Standards for Led testing. You could see how they can compare to regular lighting.

Lockedin said:

Knowing what a light will deliver at the canopy (X distance) will tell determine many of a given fixture are needed for strong, even lighting across the canopy footprint.

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Glad you guys are fluent in the stoner dialect...