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Hi all,
I've been working on setting up my new tank (120 gallon tall) and have been thinking about the differences in lighting. I know that for years we have always rated plant lighting requirements in Watts/gallon, but this is actually misleading IMO, especially when considering newer types of lighting. Here's my thinking and maybe some of you can tell me if it's right or wrong. Plants don't care about watts, but rather their lighting requirements should be in lumens. The age-old formula using watts/gallon was based on standard T-12 bulbs. A four-foot T-12 bulb puts out ~2500 lumens on average (new) and it uses 40 watts of electricity. In order to get 2 watts/gal on my 120 gallon tank then, I would be talking a 6 bulb fixture, which would put out 15,000 lumens. Using a T-5 HO bulb (54 watts @ 4 ft)I could get the same amount of lumens with 3 bulbs, but following the w/g formula would say that I need 4.44 bulbs.
I opted for a four bulb T-5 unit with 2 - 10,000K bulbs and 2 - 6500K bulbs basically due to the size of the fixture. (With a six bulb fixture, I'd have to remove the light or raise it up too high in order to open the front cover on the tank.) based on Lumens, that would be equivalent to eight T-12 bulbs (320 watts) but would only use 216 watts of electricity (rates in CT are second highest in the country).

So, after all that rambling ... is my thinking on the right track, or will I be limited to lower to medium light plants here?

Hi,
Yes, with today's technology and equipment that is available
the Watts/Gallon is beginning to become less and less used.
It was built around technology that is 30-40 years old.
However, in its defense, it gives folks a place to start
when trying to plan a planted tank. Your idea to use
the lumen value has merit but IMO, only if you place your
light meter on the substrate with the tank full. Then you
know how much energy is actually reaching the plants.
If you look in a catalog and find the number of watts for
a specific bulb and decide you need 6 of that bulb to
obtain the desired number of watts/gallon for the plants
and then read another column that tells you the lumens
per bulb, then you can do the same with the other types and,
as you mention probably purchase a bulb or two less fixture
for the same WPG. That would save you $$ which would
probably be a very good idea.

The T-5s or CFLs would be a good idea, and if you have
the $$ then an LED fixture would be even a better idea.

Frank

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