[elif]

who cares about initial costs when you're designing a sustainable system? If you spend $5k / yr on groceries, are you really going to be upset about spending $1k on lights 10 years later?

But to answer your question directly, the efficiency of LED vs CFL in terms of lumens per watt is about 2:1, so on that basis alone the payoff of running for 14 hours per day is about $8 / year / bulb. However, since the bulk of fluorescent output [0] is in the useless trough of photosynthesis sensitivity [1], it is closer to 4:1, so the payoff between your linked $5 bulb and a $35 PAR-tuned grow bulb would be a little under 2 years.

[0] CFL output - https://goo.gl/images/kF1dvH

[1] PAR sensitivity - https://goo.gl/images/dY40YW

[frgtpsswrdlame]

> spending $1k on lights

This is about buckets...

> [0] CFL output - https://goo.gl/images/kF1dvH

That's 2700k, mine are 6500k, here's the right graph (check that website lol): http://howtogrowmarijuana.com/wp-content/uploads/2014/02/GE-...

Anyways! Here's my calculation. 4 of my cfl's for $10, four of these: http://www.homedepot.com/p/Philips-100W-Equivalent-Daylight-... for $28. The cfls use 23 x 4 = 92W while the LEDs use 14 x 4 = 56W. That 36W difference at $0.12 per kWh will take 4166 hrs (~300 days at 14 hrs/day) to make up the initial cost difference. I googled up weed growth, looks like about 72 days so you can get four pot plants through this thing before you start to regret the CFLs.

[mikeyouse]

You don't use daylight LEDs to grow plants though. The "Green" spectrum is completely wasted. I don't know about weed (I was growing algae in a photobioreactor for a time) but you need a single-wavelength red LED to get biomass growth and blue LEDs to make the thing 'flower'. By using only the wavelengths that photosynthesis requires, you save a substantial amount of energy per photon. It was more of a concern for enclosed PBRs but not having to dissipate all of that heat was a big boon for energy saving too.

[frgtpsswrdlame]

So this is actually interesting but it seems that algae is different. I found this subreddit which seems to be the home of a huge spacebucket nerd. Anyways he's got citations but I'm also not experienced enough to really evaluate them. Here's the interesting parts:

>If you find a chart with a deep dip in the green area then it's for some sort of algae or bacteria, not green terrestrial plants. If you find a chart with a bunch of chlorophyll and other pigment peaks then it's only valid as an extract in vitro (in the test tube or cuvette) and not in vivo (the living leaf itself). The pigment peaks can differ depending on the solvent used and the charts do not tell how much there is of a particular pigment so take them with a grain of salt. They are only valid for the particular set up used. As a warning the Wikipedia page on photosynthetically active radiation uses these incorrect charts. Most biology text books get the above paragraphs wrong by not showing the McCree curve while also not articulating how our eyes perceive different colors/wavelengths of light. This is a well known problem with botanists who specialize in plant lighting and causes a lot of confusion and misconceptions. Even Botany for Dummies gets it wrong which is otherwise a very good book.

>Here is a spectral reflectivity profile of a high nitrogen marijuana leaf (Jack Herer). About 90% of the green light is being absorbed (it's on an 18% reflective gray card used in photography) although many plants may be closer to 80% absorption. Plants can use green light and at higher lighting levels green is more photosynthetically efficient than red (pdf file). All the latest research and my own experiments back this claim.

https://www.reddit.com/r/HandsOnComplexity/comments/17nxhd/s...

What do you think? You seem more experienced here

[mikeyouse]

Thanks for the link, I'll check it out in more detail in the morning. I worked with an algae biofuels company so for fun we messed around with a lot of different light types with various strains. We got some crazy growth rates using a flat PBR with ~3:1 red:blue leds and no green whatsoever. Our best outdoor open pond results were about 25G/m2/day (on a 30cm deep, 1/2 hectare pond) but with the right leds we could do the equivalent of several hundred g/m2/day. We had good luck with high frequency pulsing too which further cut the energy usage.

Edit:

That user definitely seems to know their stuff, all the math and logic around quantum flux and measuring photon output seems solid. It seems that green light is more beneficial in the presence of white light which makes sense from what I remember of photosynthetic apparatus (I left that co. ~2 years agoso I'm definitely rusty). Id be curious to know why the green response curve is so different between algae and plants but perhaps an investigation for a different day.