Are you talking about high pressure sodium or low pressure sodium?

They don't put out a lot of "light" per watt of power; they put out a lot of "lumens" per watt of power. Lumens are rated on how useful the light is to human eyes, so LPS gets an unnaturally high rating. Obviously, human eyes have very different needs from plants. LPS don't put out an exceptional amount of light per watt -- just lumens. And they're low PUR per watt.

HPS is fine for growing plants; it's better for flowering and fruiting rather than vegetative growth, although you can do both with them. You can grow plants under LPS, but not very well. They end up chlorotic with starchy leaves and generally poor growth.

yellow isn't that bad spectrum-wise,
but you really want a full spectrum and
for that HPS is not very good. You could
use hps to supplement natural sunlight or
fluorescent. The best full spectrum for the
money is CMH (ceramic metal halide). These
come in several types, my favorites are teh
25 watt self-ballast with PAR
100 cmh w/e-ballast
400 cmh w/magnetic ballast