I bought a 3 gal tank that has a small 10W halogen light on it. Will I be able to grow some plants in it? It adds up to 3.3W/gal which would be OK for fluorescents but I don't know about halogen.
I don't see why not. Halogen lights may not have the ideal spectrum (means I dont know), but in the end its all about intensity.
The WPG dosnt apply here though: It is good a good rule of the thumb for 2o - 5o gal 'common' size aquariums using T8, but in most cases not applicable today anymore (in my opinion).
10W of halogen light probably won't grow much of anything, as a halogen is just another type of incandescent bulb. 10W of incandescent light won't grow much of anything very well.
You could try some very low light plants or floating plants, but I wouldn't get my hopes up.
I've had the tank running for one month and the 10W halogen is working well with plants. I put on lights from other aquariums to test the difference and it's significantly brighter than a 15W incandescent bulb and just a bit dimmer than a 9W fluorescent. The light color is also between the two types. I have a crypt, cabomba and some green hedge and they're growing very well (maybe too well for such a small tank). I'm happy with it.