40 watts. The comparison to incandescents basically means that the bulb will appear to put out as much light as a 75w incandescent. For our uses, you need to go with the actual wattage of the bulb. While wpg
is probably OK for a 10g, when you get into larger and deeper tanks, it's not really a good equation to use. One other problem with the screw in PC
's is that a lot of the light is reflected back into the bulb. This shouldn't pose a problem in your tank, but again, it's something to think about when setting up a larger tank. I would think that 40 watts would be plenty of light for a 10g with plants...but then I'm not the plant guy here.