I always thought it was 35 watts was the minimum & up to 50 watts per sq. ft. max.
Watts is a bad indicator, I remember the Frank/Rosenthal bible we all grew up on stating 10 watts per square foot minimum though 20W is recommended .. if you're using the old fluoro tubes.
HID wattage will be different.
Now look at this table for various LED's
The best small grow light for a 2 x 2 or 0.6m x 0.6m grow tent
The SE-1000 and the TS1000 both use the 1000 nomenclature but look at the actual W they draw, 103 and 146 respectively.
Before you think the more watts the better, the TS1000 draws 146W but it has a usable PPF of 286
Now look at the Migro Aray 2 PRO, it draws less at 130W but has a higher usable PPF of 308.
Other variables also come into play, how reflective is your tent, how high is your light from the plant canopy. The wattage is just not that reliable.
PPFD is the best standard to use, measuring it at different parts across your canopy, it eliminates inefficiencies in light design across various brands/models.
PPFD is the amount of light photons that hit a surface (your plant canopy) in one second. It’s measured in units called micromoles (μmols). Simply speaking – it’s the strength of the light that reaches your plants.
Cannabis seedlings require only 200-300 PPFD per second during the first three weeks of life.
You simply need to double the numbers above – 308 PPFD to 616 PPFD during the vegetative phase and 463 PPFD to 925 PPFD during the flowering phase.
If you’re growing autoflowers you should stick to the recommended vegetative phase light intensity of 306-617 PPFD for the whole life of the autoflowering cannabis plant.
From my previous posts above showing PPFD charts, you'll see how it's distributed more evenly the higher the light is, preferable for seedlings, at the same time you may also lower the intensity via the dimmer if required. With even distribution you can try growing multiple seedlings and prune out the weakest, slowest growing as you go.
More reading here:
Understanding Grow Light PAR, PPFD, Wattage and DLI
So how much light do your plants need?
The DLI is when you take those PPFD numbers and add them up over the course of one day.
DLI stands for Daily Light Integral. This is the amount of light fruit, vegetable, and cannabis plants need each day to produce fruit vegetables or marijuana.
Below is a link to a simple DLI calculator, plug in your plant's age in weeks, hours of light, PPFD intensity and it produces your DLI, letting you know if it's adequate.
You can still achieve the same DLI while varying the parameters, the less hours the more PPFD required and likewise if you increase your hours you can lower the PPFD (via dimmer or raising the height of the light from canopy) and still get the same result.
Why is this useful?
Firstly, pumping too much light at your plant is wasting electricity, it can only take so much per day so don't throw away money.
The way I use it:
In winter, I can lower the PPFD and run the light overnight, which keeps the tent a little warmer during the coldest part of the night, saving electricity by not having to use a heater or running the heater at a lower setting.
In summer I can increase PPFD during the night when the temp is coldest and have the lights off during the long day when the outside temp is warm, saving me on air conditioning.
Have a go and see if your DLI result is right for your plant at its stage of life.
Photone - Grow Light Meter
Quite the post but I hope it gives you a better understanding and why you should ditch relying on wattage.
If you don't have a light meter, there are PPFD apps that are decent, otherwise track down some charts from your light manufacturer. Thanks to the work of one individual who ran some tests, you can look up his charts for some popular makes/models here.
PPFD Data
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