You’ve probably heard about umol/j and you may be wondering what it is. Umol/j is a unit of measurement for light intensity, but when we talk about indoor growing, this refers to the amount of light emitted by the grow lights in an indoor garden. A higher umol/j rating can help your plants thrive and grow faster than if they were grown with less intense light. You’ll have better yields and healthier plants!

The difference between umol/j and umol/w

Umol/j refers to the amount of light emitted by indoor grow lights. Umol/w, on the other hand, is a unit for measuring total energy output from indoor grow lights.

The difference between the two measures is that umol/w tells you how much of the indoor grow light’s energy output that your plants are getting, whereas umol/j only tells you about the total amount.

How to calculate the umol/w of an LED grow light

To calculate the umol/w of an indoor grow light, you need to take into consideration the power (in watts) and also the lumens from that indoor grow light.

The formula is:

Power in Watts x Lumens = Umol/w

Many grow light manufacturers are pushing 2.4umol/W

Why is umol/w a better metric than pure watts used by many manufacturers

While wattage alone may seem like a good metric for power output in indoor grow lights, this does not provide measurements on how your plants are receiving light from that indoor grow light.

In the case of HPS lights, most of the watts get lost as heat and not net photons. That’s how you get cooked plants. With LED grow lights, especially ones like the S-700 grow light and Phytomax 2 grow lights series, you’re pushing that 2.4umol/W boundary.

How many umol/j do your plants need?

The answer to this question varies depending on your indoor grow light type, the spectrum of that indoor grow light and what you are growing.

For example, Tomato plants need a different amount of umol/w than fruiting vegetables or flowers. If you can find one, get a grow light with controllable light spectrum and intensity like the S-Series grow lights.

This is why it’s important for indoor growers to know their indoor grow lights wattage (in watts) and also the lumens from that indoor grow light.

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