Watt conversion factor?

[mosasaur]

Looking at non-aquarium light fixtures yesterday, I noticed a lot said something like "uses 17 watts, gives 75 watts worth of light" or something similar. Coming home and looking at the flourescent bulb in my secondhand aquarium light, I see it says 25 watts but to me the bulb looks equivalent in light output to, say, a 100 watt incandescent bulb. Is there a convesrion factor that can be applied? I wnt to bring my light in that tank to 3 wpg, so I need to know how much I have now.

[mosasaur]

Never mind. I Googled, and found that indeed there is a conversion factor of about 5 - my 25W bulb is puttng out the equivaent of a 125W incandescent bulb!

[Hoppy]

The best thing to do is to forget about watts entirely. Instead, think about the types of lights - T5HO, T5NO, T8, PC, Screw-in Power Saver, MH, etc. Each of those types of lights requires a certain number of bulbs, at a certain distance, to get what ever light level you want to use. And, that light level depends on what kinds of plants you want to grow, how fast you want them to grow, and whether you are willing to use pressurized CO2 and do meticulous tank maintenance. You lose the simplicity of watts per gallon, but you gain a much better idea about how well lit the aquarium is.

There are many T5 lights available now, and some at very good prices. So, it makes sense to me to just limit yourself to T5 and possibly PC lights. At least that makes it a lot easier to figure out how many bulbs are needed for each tank size.