When thinking in lighting for plants, you need to avoid thinking in lm, because its a units for human, and gives little info about effect on plants. Its necessary use radiometric units, optical watts or better, micromols of photons (uE), because photosynthesis is directly related to photons count, not energy (both a blue photon and a red one do the same photosynthetic effect, although the blue one carry a lot more energy).
A typical 400w HPS, the Phillips SONT Plus, emits 670 uE (1,53 uE/watt, supposing a consuption of 440w, including ballast). About 20-25% of them get absorbed by reflector (and easily a 10% more if the fixture has a glass barrier). It means from 536 to just 435 uE avalaible for plants.
Say you use a LED fixture with 40% white leds and 60% red leds (the ideal would be using blue, red and white, but this is just to make calculations easier, as anyway its just an aporoximation). 200 uE provided with white leds and 300uE provided with red leds.
For example, using the Cree XR for white. The XO bin gives 311 lm/optical watt at normal operating conditions. Say you use the newest bins emitting 100lm/w, wich equal to 100/311=0,321W emited=321mW. 100lm/w emits 1,4 uE/w (14uE/Klm). So you need to install 200uE/1,4=143watts of white leds (considering optical losses negligible, although it may count for up to 5%).
The XR red, bin R3 at Tj=52ºC, brightness bin M (highest avalaible currently), gives 38,9lm (already derated for increased Tj over ambient, considering thermal resistance junction-ambient about 30ºC/W). At average consuption of 785mW@350mA, its 49,5 lm/w. At 38,4 uE/Klm, its 1,9 uE/w. 300/1,9=158w of red leds.
Total of 301 watts of leds, plus drivers losses (10% with very good drivers), 330w.
So you are saving 110w to achieve same radiance in photons. Using it 12h a day, it mean 482KWh a year. For me, its about 60€ (~80$) a year. The current cost of a 330w led system would make it impossible to compensate with that electric bill savings and relamping costs.
Im putting it in the worst scenario, as getting the light better distributed and spectral advantages (about 15-25% more efficacy of red photons against the yellow emited by the HPS) may count for a reduction in the uE required to achieve same photosynthesis. Taking this into account, and supposing 400uE of LEDs can have same photosynthetic effect than 500uE of HPS, then you will need to install 225w of leds (about 55% of required with the HPS). Electric bill savings would be of 845KWh in this scenario (about 100€~135$ a year in my case). In ten years of use, savings may count for near 2000$, allowing to recover the initial investment. But anyway, too long to considering it "cost effective".
Notice that the bins used are premiun ones, wich are very expensive when thinking on install so many watts of LEDs. When we mod a flashlight, we dont mind paying for a premiun bin, but it isnt the same when buying more than 200 high power leds. The only advantage, you dont need to do a group buy
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Said this, im doing a LED light fixture for plants, using stock cheap leds, but i dont think it worth the cost, just giving use to some old leds (2005, now near obsolete). I want to check how it work with plants, and whats exactly the photosynthetic gain of using LED photons.
If prices continue going down and perfomance increasing as they had done along this year, maybe next year or the next as much, cost scenario could be very different. I hope so.
Future is bright!
