• You are viewing the forum as a Guest, please login (you can use your Facebook, Twitter, Google or Microsoft account to login) or register using this link: Log in or Sign Up
  • You can now follow UKAPS on Instagram.

Using (2) equal wattage bulbs increase overall wattage?


10 Sep 2008
United States
Hi everyone,

I read this elsewhere on the net:

Example: Take a 10 gallon tank, by the math you could put 2 - 18 watt Normal Output Flourescent bulbs over it and have 3 + WPG "mathematically" but you still only have 18 watts of "intensity. You will not be able to grow many varieties of plants successfully in this tank even though you are at the 3 WPG that we are looking for. Now if you take that same 10 gallon tank and put a 1 X 36 watt Compact flourescent over it you now have the same amount of watts total but you have 36 watts of "Bulb Intensity" in which you can grow virtually any plants you choose.

What does that mean? Can anyone explain to me in layman's terms what this guy is saying?


Expert/Global Moderator
11 Jul 2007
Chicago, USA
It means that "the net" is thoroughly confused. If someone thinks they can't grow plants with 36 watts of T8 over a 10 gallon tank then they need to study more about light and more about plants. Maybe they need to study more about CO2 as well. :wideyed: Here's a question for you: Do you think algae can tell the difference between two 18 watt bulbs and one? Lets go extreme: What about if I light theat10G tank with 10 of those bulbs. Do you think there'll be a difference? Let's get real.

It might be helpful to think about light differently than the intuitive way we have done our whole lives. Have you ever looked at a movie reel? One second of moving pictures has maybe 25-30 individual frames. If there were less frames per second the movie would look choppy, sort of like those oldie films. When we perceive light we see a smooth and continuous image, but that's just because there are so many "frames" per second. Light is actually made up of trillions of individual frames. We call those frames "Photons". So when light falls on the the objects we see it's much more like a rain shower. Trillions of photon raindrops fall per second upon our eyes so we see a continuous image.

This is not how a plant sees the world. The plant can actually count the individual photons and it responds to the total number of photons striking the leaf surface every second. Each photon that strikes a chlorophyll (or other pigment molecule) causes a reaction, typically an excitation which causes ejection an electron. These are the electrons that are used to power the chemical reactions in the plant.

Now, can you see that adding twice as many bulbs causes twice as many photons to strike the chlorophyll molecules per second, which causes twice as many electrons to be liberated, causing twice as many chemical reactions? Does this seem reasonable? Doesn't the same thing happen to your eyeballs? If half the bulbs in your lounge are blown, even if they are all the same wattage, can't you sense that without looking directly at the bulb? Of course you can. There will be less light in the room. One doesn't need to be Albert Einstein to figure that out. It was Albert Einstein, by the way that discovered and described the phenomenon of the Photoelectric Effect, i.e. the relationship between electrons and photons, for which he won the 1921 Nobel Prize in Physics.

The only way to compare 2 X 18 watt T8 bulbs to 1 X 36 watt T5 bulb is to count, for each, how many photon raindrops cross a unit area, per second, at a specific distance from the bulb or from some reference point. You can only do this with an instrument called a PAR meter. PAR means Photosynthetically Active Radiation. This measurement is also known as the Photosynthetic Photon Flux Density (PPFD). Now when you make these measurements it could very well be that the single 36 watt T5 has a higher photon flux density than the double 18 watt T8. A lot will depend on the characteristics of the bulbs/ballasts themselves. But if so, it won't have anything to do with "the nets" explanation, that's for sure.

Hope this clarifies.