Wattage means light output and not power use, right?

[kenyth]

When wattage is referred to as a measurement to use for plant needs, I assume they're referring to the light output right? A flourescent grow-light of 17 Watts (as the meter runs) has a light output rating of 75 Watts. This confusion occurs because of the inefficient incandescent bulb being used as the benchmark. A 75 Watt incandescent bulb puts out the same light as a 17 Watt flourescent tube, but gives off 58 Watts of extra heat as a bonus.

[sergo]

lumens is the term i think you are looking for.

[kenyth]

Yup, it is. But I was just wondering if when folks say "You should use two watts per gallon.", do they mean the light ouput rating, or actual power use rating of the flourescent bulb?

[sergo]

i think a lot of people get confused with wattage (power) and lumens (light output). 2 bulbs can have the same wattage but different lumens. all you have to do is check the box the light bulbs come in as they have all of the information on them ie. lumens, wattage and spectum.

[Chrona]

Yup, it is. But I was just wondering if when folks say "You should use two watts per gallon.", do they mean the light ouput rating, or actual power use rating of the flourescent bulb?

Thats a good rule of thumb, but to expand on that:

-That rule only applies for regular fluorescent light bulbs. Incandescent does not count because it is so inefficient.

-That rule does not apply to smaller tanks, because there is a minimum threshold of total light output for a plant to grow. Ie a 4 watt CF bulb would do jack over a 1 gallon tank, even if it does technically give you 4 watts/gallon.

-The exact opposite applies to larger tanks. In a 55 gallon tank, 2 watts per gallon (110 watts) will grow many many plants, while 3 watts per gallon will grow almost any plant you want. This is because, again, the intensity of the light is so much greater with a 110 watt fixture.

-A better rule of thumb is the wattage per area.

-Contrary to popular belief, depth of the tank does NOT count towards your consideration of wattage, unless it is over 30 inches. Of course, it DOES if your taller plants overshadow your lower plants, but that's something entirely different. Light penetration in 30 inches of water is almost identical to 18 inches. This line of thinking was imported from saltwater tanks, where corals, etc are usually in 20-30 ft deep water, where light intensity and spectrum IS affected by the depth of their natural habitat, and thus a consideration when keeping them in an aquarium. The only time this rule not applying is with a poorly designed/inefficient reflector that scatters light instead of concentrating it. Unfortunately, this is sometimes the case, even with higher quality fixtures.

Hope this helped. All that reading wasn't a waste I guess.

[Chrona]

Oh and wattage is how much power the unit draws from the wall. In general, fluorescent bulbs will generate about 60 lumens per watt, while incandescents generate 10-20 lumens per watt.

[sergo]

good jod Chrona :thumb: