CFL wattage confusion

[Mixer]

Ok, so I am trying to get screw in CFL's for my 10G planted tank. I know that I want between 6500K and 10,000K color temp, however I am a bit confused about how to calculate my wattage.

I started looking at my hood, and it says 30 watt maximum, ok I get that. So I start looking at CFL's and they all say something to the effect of 26watts = 100watts, ok so as far as my hood is concerned 26watts, cool.

What about my wpg calculation, is it 100w or 26w? I'm leaning toward 100w for this calculation (I hope). I currently have 2 13w = 40w daylight 5000K CFL's, so is that 80w = 80/10 = 8wpg ?

Also, when someone says "you have low/medium/high light" do they mean the color temp, 10,000K = high light, or the wattage, 5wpg would be high? So is my 8wpg formula above low light because of the color temp, or high because of the wpg?

Thanks in advance for all the help.

 

[Vircomore]

1. The wattage conversion you're looking at is "Fluorescent Watts" to "Incandescent Watts". So a 26w Fluorescent is equivalent to a 100w regular light bulb. HOWEVER.

2. The wattage calculations on aquariums are done by Fluorescent Watts and not Incandescent Watts. So if you have a 26w CFL over your 10gal tank, you have 2.6wpg. Which really isn't that bad. It's enough for low and moderate light plants, but you'd probably need a whole different socket to go over 30w for high light plants.

3. If you're looking at the type of CFLs that screw into a screw-in lightsocket - it is recommended that you get the bulbs that are "double-U shaped" instead of the spiral, because it has been found that you actually lose quite a bit of light from the spiral's shape.

4. Low/Medium/High Light plant calculations are done by watts-per-gallon (and also depth - plants in a deep tank are farther from the light than a shallow tank). As long as the color temperature is between 6500K and 10000K, the lights will be effective for plants.

 

[BigJim]

The equivalent incandescent wattage number is useless. Use the smaller number. If you have two 13W CFLs, you have 26W of light or 2.6 Wpg over a 10g. The Wpg rules don't apply well to small tanks like a 10g, so you're probably in the med-low to medium light range.

The low/medium/high range refers to the bulb wattage, not the color temperature.

 

[Mixer]

Ok great! Thank you very much for the replies, so 2.6wpg it is! So what does the scale look like for low/med/high? since BigJim says 2.6wpg is med-low to medium range, is 1.5wpg low , and 3wpg medium, and 6wpg high?

I do understand that the distance the plants are and the depth of the tank matters, I am just trying to figure out a general rule, so I might have a place to start for getting plants that have a better chance of survival.

 

[BigJim]

It's typically 1-2 Wpg = low-light, 2-3 Wpg = med-light, and 3+ Wpg = high-light as a rule of thumb for fluorescent lighting. Tanks smaller than 20g and larger than 90g don't follow the rule so well. You need more wattage per gallon over a small tanks and less wattage per gallon over a large tank to reach the same lighting levels. Reflectors make a big difference and T5HO lighting further complicates things.

Light is a very complicated entity. Quantifying it accurately requires a lot of expensive equipment and the vast amounts of data would blow most peoples' minds.

I can grow med-low to medium-light plants very well in my 20L with two 18W CFLs. High-light plants won't make it though.

 

[Mixer]

thanks BigJim for clearing that up for me. I guess for now I am ok with my 2.6wpg @ 5000k , if I can at least keep these few plants alive I will look at upgrading to a 6500k or better CFL