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Question of the Day Lighting - are we getting back to watts per gallon?


Staff member
Founding Member
  • Joined
    Oct 29, 2022
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    Miami, FL
    When I started in the hobby, it was VERY common for people to tell you that you needed a certain watts per gallon. Low, medium and high light were all defined by watts per gallon. Mind you, we mostly had fluorescent lights and we envied those that could afford the metal halides that were uber powerful. I used to dream about the Dupla mercury vapor lights (yes, similar to those that light up stadiums).

    Over time, as products changed and improved, we started to focus more on photosynthetically active radiation or PAR. This made absolute sense as we know this is what is important for photosynthesis. Products are now (mostly) defined by PAR output. We also began to focus on the light spectrum that is optimized for photosynthesis. It's no longer about electrical output or full spectrum white light. It's about PAR and spectrum.

    Flash forward to today. Given current LED technology, spectrum is almost a given. We are no longer looking for the right spectrum for photosynthesis but for the spectrum that, additionally, is most pleasing to use (e.g., RGB). More importantly, we are chasing PAR numbers, especially at the substrate level, if you are going for the freedom to grow any plant you want.

    I've noticed that LED lights now have a power consumption indicator that relates with the light output. So, for example, at 50% or so, my Netlea LED is at about 96W. I'm probably going to have to increase that to get the amount of PAR I want in the tank. Therefore, power will also increase. In the end, I will be back to watts per gallon. I will need roughly 3w+ per gallon of LED power to get the PAR I want.

    Are we now going back to defining our equipment and needs with watts per gallon? Or, am I off?
    I’m not sure. I’m not from that time and just research for PAR at depth. I guess in my mind I’m learning to convert watts to potential PAR expectations.
    I judge prettymuch everything just by the state of my plants. Too much light I am usually running into algae issues. Too little light I am usually noticing lower stems struggling, less colorful growth, etc

    I think most fixtures now come with PAR charts out of the box? We should just go by that or rent PAR meter if you really care
    I think there is no cookie cutter PAR recommendation besides the general low, medium and high recommendations. You really need to see it in action in your tank and then adjust as @klibs says.