New to planted tanks. Question about CFL bulbs. - The Planted Tank Forum
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post #1 of 7 (permalink) Old 03-08-2015, 01:51 AM Thread Starter
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New to planted tanks. Question about CFL bulbs.

I have a 20g tall and I am planning on doing a planted tank. I have a fixture that I made and I was thinking about putting in 2 26W 6500k CFL bulbs. These bulbs replace normal 100W bulbs, does that mean I will have the equivalent of 200W shining into my aquarium?

According to the Watts per gallon rule, that would be 10W per gallon, which is overkill if I understand correctly.

Or does it still only add up to 100W with both bulbs equaling 5W per gallon, which is more acceptable for plants that require higher light?

Or since they're 26W does that mean I will have 52W shining into the tank which would be 2.6W per gallon, which is good for low to medium light plants.

It's all so confusing. Sorry if my question isn't clear.
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post #2 of 7 (permalink) Old 03-08-2015, 03:09 AM
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Back a long, long time ago in a land far far away when men were men and women were, well women and everyone got along there was this sphere that had a thing called a "filament". Now this "filament" had a weird eccentricity. If something called "electricity" were put through it it would get super red hot. Some would say it even glowed.
Now when it was all aglow it would consume that thing called "electricity. But it was kind of selfish. See if you put one "watt" into it gave you a certain amount of light and no more. So it was one "watt" in and one "watt" of glow appeared. This became a standard.

Now along comes this girl. Her name is Fluo. Fluo is a little more considerate. She said I am going to take a certain gas and put power through that gas and I am going to make it glow. I will now give you one hundred of those "watts" of light but I am only going to have to put in 26 of them through my nose.

LEDs are even more efficient.

OK A resistive load like a light bulb gives you a certain amount of light. One in one out. With fluorescent that isn't so . There's a whole bunch of theory and the physics of it but that bores even me. Suffice it to say what the wattage of a bulb is only what it draws. An incandescent and its output is the standard. When a fluorescent gives you 100 W output because of how it works it only requires 26 w.

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post #3 of 7 (permalink) Old 03-08-2015, 03:27 AM
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I have 2 23 watt cfl bulbs in my 50 gallon tall and I grow things perfectly fine .... I think you'll only need one bulb tbh ... 2 is over kill ... Usually people that have longer tanks will need multiple bulbs because of the length factor ...
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post #4 of 7 (permalink) Old 03-08-2015, 03:32 AM Thread Starter
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So I should go off the 100 watts and not the 26 watts. Does that mean two 100 watt bulbs equals 200 watts in the aquarium? Thanks for your reply.
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post #5 of 7 (permalink) Old 03-08-2015, 03:38 AM
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Long story short... It's not that simple as using WPG. But you're closer with the 2.6 WPG estimate.. but the real answer could be significantly lower or higher depending on your setup.

WPG or anything watt-based is a fundamentally stupid way to measure lighting, as it inherently depends on what kind of lighting you are talking about, how efficient it is, and how directional it is. Your plants don't care about how many watts you burn, they care about how many photons hit their leaves in a given period of time.

However, WPG estimations are easy to work with, and can work OK, assuming you are dealing with only one type of lighting.

The WPG rules you are quoting are designed around T12 or T8 bulbs in a "shop light" or similar fixture with white-painted flat reflectors. Assuming you are dealing with this kind of lighting, you can make general estimates of how much light density you are creating per watt consumed.

Your problems trying to apply WPG to your scenario are:

1) CFL bulbs are designed around newer fluorescent technologies than T12 bulbs, and thus produce more photons per watt.

2) your fixture is certainly not a "shop light", so it may be focusing that light more or less efficiently than a shop light does. A 20 watt CFL bulb just hanging bare above the tank sends most of its light out into the room. Put it in a broad white-painted desk light fixture and it will put more into the tank, but probably has a lot of spill... Put it in a polished parabolic reflector, and that light can be focused down into a very small part of your tank. These three scenarios result in very different levels of light hitting your plants.

This is why we measure light in PAR instead of Watts, but that does make things more complicated...

Generalities of PAR vs Watts:
Lighting an Aquarium with PAR instead of Watts

Some par measurements of spiral CFL bulbs (see post 21 in particular, also note that authors idea of what constitutes low/medium/high light are quite different than Hoppy's idea of what those ranges mean.. personally, I think 40-70 PAR for "low light" is absurdly high. Those ranges pretty much require CO2 injection to avoid algae issues)

PAR Data-Spiral Power Saver Bulbs, lighting question

Originally Posted by FreakyFishes View Post
So I should go off the 100 watts and not the 26 watts. Does that mean two 100 watt bulbs equals 200 watts in the aquarium? Thanks for your reply.
No... do NOT go off the "incandescent equivalent"...

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post #6 of 7 (permalink) Old 03-08-2015, 03:41 AM
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Originally Posted by FreakyFishes View Post
So I should go off the 100 watts and not the 26 watts. Does that mean two 100 watt bulbs equals 200 watts in the aquarium? Thanks for your reply.
The old "watts per gallon" rule was based on fluorescent lights .. So the "efficiency" was built in..

The 26=100W is for florescent equiv. to "light bulbs" i.e tungsten filament types.
You "use" the 26W number, ignoring the "equivalency" number..

Oh and see above..

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post #7 of 7 (permalink) Old 03-08-2015, 05:18 AM
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Check out that thread for a pretty decent sumation of your scenario, long story short it depends on orientation of the bulb, reflectors, height above tank etc.

In practical terms I have 2 13w cfls on a 10g tank with no issues, no co2 or ferts or anything, your milage may vary.
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