What kind of light source and what print size, and what distance between the light source and the contact frame are you aiming at?
For carbon I try to aim for around 100W for 4x5" up to around 30cm distance using 365nm or 400nm LED, and around 300-400W for 8x10". That's probably more than you'd need since carbon can be pretty slow.
You can probably get away with 50w to 100w for 8x10" size or thereabouts. 50W is a bit on the meager side though, IMO.
So it boils down to: it depends...