Sorry, my math is off.
Two years on and this topic is more timely than ever.
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Aye. I think we’ve learnt though that it’s not about watts, but more about the light intensity on the subject and how it’s spread over the spectrum that counts. The next 5-10 years are looking to be very exciting for the indoor horticulturalist imo. Me, I’m still trying to work out if i should use a flash to take photos at dusk or not 
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Within 2 hours of lights off should be fine. just don’t make a habit of it.
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I would only use it within 10 minutes of lights on or off. It will be fine at that point.
The statistic to keep in mind is PAR Watts per square foot. 15-20 is the target. so the way to calculate this is that if your LED light is 50% efficient you multiply x 50% and that’s your PAR wattage.
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