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watts per gal

questions about the whole "watts per gal" idea in tank lighting.

I'm not an electrical engineer, a mathematician, or a botanist, so what I'm
thinking is most likely wrong, or something like that :)

Numerous websites have formulas to calculate how many "watts per gallon" of
light you need to grow low-mid-high light plants...

The questions I have are:

Isn't watts a rather unreliable measure of light output?  I mean, a 75 watt
standard incandescent puts off as much light as a standard 20 watt cool
fluorescent, or a 13 watt compact fluorescent twist-bulb.  Which wattage do
the formula's speak of... I assume they mean watts fluorescent (since that
to be the most popular)?

Are there any formula that use something more related to actual light
output, like lumens or candlepower?  Or formula that are calibrated to the
wattage to light output ratio for standard fluorescent bulbs (can't afford
the power compacts yet).