Watts and lumens for growing marijuana


A question from a fellow grower:

I was wondering if you could respond or publish the equivalency of lumens to watts as everything measures output in watts but engineering wise an inefficient light can use MORE watts than is worth the lumens…so as the lighting technology grows more efficient (encouraging reduced wattage for same high lumens) we need to factor in lumens and not in watts, or at least be able to make the comparison… that being said - can you tell us in terms of LUMENS instead of watts how much yield to anticipate?


I would have a whole lot to say about this and so I might have to get back to this with a more detailed answer that actually answers some of your questions, but in the meantime you might want to look into photosynthetically active radiation(PAR), because lumens won’t really be the best way to determine what you are trying to determine.

Watts would be the more accurate number overall than lumens because the goal of any light is to try and turn 100% of the electrical watts into 100% PAR watts, or 100% of the type of light watts that a plant can use, which would mostly be in the blue and red spectra, but this almost never happens as a lot of the electrical watts is lost in heat energy(infrared light/radiation) and or other less useful energy such as green light. Watts is a measurement of energy, and energy can exist as matter or any of the forms of the electromagnetic spectrum, i.e. electricity, visible light, as well as all the rest of the radiated spectrum – heat/infrared light/radiation, uv radiation, gamma rays, x-rays, etc. It can get quite complicated.

And really most people use yield per area more than yield per watt anyway since most people are going to try and put as much light per the area as they can anyway. Everyone will be trying to get around 30 - 50 watt per square foot no matter the type of light as this is the industry standard. And this is all an attempt to replicate the sun anyway, and so for example if you have the lights set up correctly for your area you should get similar yield per square foot as you would per square foot outdoors. Making yield per area the best way to anticipate yield for a given growing area, indoors or out.

The proverbial “gram per watt” is a goal more than a way to estimate or anticipate yield.


Mr. Greens book " The Cannabis Grow Bible " explains Watts vs lunens I know because I have read the book 10m times over…lol


converting lumens to watts is not simple because it depends on the spectrum that a light produces one watt or radiant power will produce 683 lumens at 555nm but only 20.5 lumens at 445nm. a light can have a high lumen rating and still be very inefficient. it would appear bright to us because of the way the human eye response to color but convert much of the energy it receives into heat. LPS lamps are one example of this.the cooler a light operates for a given input power the more efficient it is at generating light just compare incandescent lights to fluorescent.one other factor that needs to be considered is the environment plants were breed in.plants adapt to their environment and after many generations of breeding under HPS lamps indoor strains have adapted to this form of light. our understanding a plant light needs are constantly evolving http://www.heliospectra.com/sites/default/files/general/What%20light%20do%20plants%20need_5.pdf yield per watt is dependent on a lot of factors including strain and growing style.each type of grow light requires a unique growing style to produce maximum yield but 0.5 grams per watt seems average for new growers and experienced growers are getting 1.0 grams per watt in many cases


True enough. Photosynthetically active radiation(PAR) is the only truly accurate way to know exactly what you are giving your plants or what they need. Unfortunately green light looks the brightest to our eyes, obviously the green light is the least needed by the plant, so a light that looks the brightest to us might not be the brightest to the plant as it might not have much red or blue light.