i had a power compact flood light on my fuge...couldn't even tell if anything was growing (using a 2700 kelvin bulb). so i put a different power compact (the spiral kind) over the sump and my baseball size clump of chaeto has doubled in about 10 days. the new bulb is 6500 kelvin, consumes 27 watts (but is equivalent to a 100 watt bulb) and was bought at Home Depot. it comes in a blue package and is manufactured by Consumer Electrics. they also sell a spiral compact in a green package, but i believe the green one is rated at 3000 kelvins.
long story short, buy a bulb that is 6500 kelvins with the highest PAR (36 or 38 i think) you can find.