I've spent the day reading up on the latest LEDs and comparing them to traditional HID lights. I thought I'd share my thoughts and would be interested to hear others comments. Skip to the bottom for a simple conclusion.

First what units to use. Lumens are often mentioned. they measure total light output. That is across the entire spectrum and not caring how the light is focussed. so if we *roughly* want to compare lumens quoted from HID lights and from LEDs we need two fudge factors to make things fair.

Firstly HID lights waste energy lighting parts of the spectrum that plants just don't use. for example green light, plants reflect that away which is why they look green. LEDs can be chosen to only emit energy in parts of the spectrum plants do use. Looking at a graphs of wavelength vs effectiveness for photosynthesis and also vs relative light output I'm going to say that HID lights waste half their energy in the wrong wavelengths. so we can double LED's quoted lumen's in order to make a fairer comparison.

Secondly HID lights are hot, so have to be far away from the plants. Light intensity falls of quickly as you move away from a light source, although this can be mitigated with reflectors and reflective sheeting. LEDs radiate negligible heat so you can put them close up so all the light goes on the plants. Having multiple LED light sources also means you can arrange things to waste less light. Again I'm going to say half the light is wasted with HID lights scattering light about. So we can double the lumen quote of LED's again to make a fairer comparison.

This means we have a final fudge factor of 4. That is we can multiply the Lumens emitted by an LED by 4 to give us a new unit of Lumens adjusted for wavelength and source distance. I know it looks like I've just made up the numbers and I have to some degree. What I care about is that the numbers are the right order of magnitude. I'm not aware anything else that greatly affects the comparison of LEDs and HID lights. So although my assumptions are probably wrong they won't be *very* wrong. I'm confident the correct multiplier to compare LEDs quoted lumens with HID lights is around 4 not 40, 10 or even 1.5.

Right so now I have decided on a fudge factor of 4 lets put it to some use. The very latest Luxeon Rebel LEDs have a quoted output of 110 lumens (impressive!). However for the coloured ones we are interested in it's 85 for red and 48 for blue. Lets smooth the maths again and say about 60 lumens per LED and with our fudge factor of 4 that is equivalent to 240 lumens adjusted... Lets compare this with a 400w HPS light of around 50000 lumens. 50000 \ 240 = 208 200. That is you need about 200 LEDs for equivalent growing power of a 400 watt HPS.

The high power LEDs are about £2.50 each but you also need heatsinks, PSU's and other bits and pieces so call it a fiver.
£5 x 200 LEDs = £1000 for LEDs vs roughly £100 for a 400 watt HPS complete setup. so money wise LEDs are *about* 10 times as expensive as HID lights, at least for initial outlay. The High power LEDs need about 1 watt each. so with 200 of them they are only *about* twice as efficient as an HPS light as far as power consumption goes.

Conclusion: LEDs are currently 10 times the price but only twice as efficient as HPS lights (this is for an optimum setup using the *very* latest LEDs, all of the commercial LED kit I looked at came of *much* worse). Also they are still not well understood for growing and can be very difficult to set up. Only a madman would do it. Fortunately there are plenty of madman around as we need research because they will be the future. especially for stealthy small grows.

P.S. if you think this is wrong show me why, that will make me happy

Hi mate,

HPS at present makes leds look like pretend lighting really.

You can go on about P.A.R.,heat given off and a host of other things but unless you are into growing 5 gram lollipops it's best to stick with hps.

There have been some very good experimental grows done here already and they do loads for led geekness but don't bring home the canna bacon.

Yes thats what I said if you had even just read the conclusion. Nullius in Verba As a geek I dont like to just take peoples words for things. I wanted to find out for myself what the current state of play with LEDs was. I shared my findings *with some figures to back it up* so that others might not waste time or money on LEDs without knowing what they where getting into.

I say again: you would need to spend a grand on cutting edge LEDs (not ready made kits) to come even close an HPS. With the extra cost, effort of wiring up and risk associated with new tech your motivation would need to be something other than "bringing home the canna bacon"

i'm just deciding on what to use for my next project. i came to the conclusion that 5mm leds can be 4 times more expensive for the same power consumption than high power (i use HP) leds. so i decided my next setup will be made of HP leds. i'm looking at luxeon for quite a long time and i'm almost sure i'll use some of those. rebel model is very small and my guess is very difficult to handle with. i'm looking at luxeon K2 royal blue with typ 575mW @ 1A and luxeon III red with typ 140 lumen @ 1A. i most probably will put another small ammount of cool white with typ 100-140 lumen @ 0.7-1A for the minimal ammount of spectrum that i won't cover with blue and red leds. not a long time ago i read about white HP that is expected to put out 1000 lumen. maybe if i'll have to wait before i start the project and if they release that model soon enough, i'll use one of those (if the price won't be astronomicaly high).
the only thing i'm affraid of when i'll make that new setup is the heat problem. i'll first try to make a passive cooling with Al or Cu cooling body and attach the leds to it. if the temperatures will be too high, i'll try to cool that metal with some PC fans. if the heat will still be the problem, i'll make a water cooling. it's not that complicated to do that.
IMHO it's not the question IF but WHEN the led lighting will replace hid lighting.

I did a lot of reading and I am sure the luxeon rebel are the best value they are small and hard to handel but you can buy them premounted on heat effiecient PCB's from germen site led-tech. they also sell them heatsinks that you can glue to the back of star PCB's.

I was thinking this morning that given how much brighter the white LEDs are it might be worth not trying to match the wavelength and just use white. They are three times as bright I think that might outweigh the advantage of using only wavelengths plants will use. The white LEDs available in more premounted configurations.