It depends on a lot of factors, not the least of which is the acoustics of the room/stage.
These days I usually carry my Ashdown LB30 (30W tube head) and an older EA 2-10. They get the job done plenty. I do put the whole thing on a mixer stand to the 10s are hitting me in the shoulder blades while I'm standing directly in front of it. But even 15 ft away when I'm singing I hear it fine.
Watts alone are only a small piece of the pie that will determine if your rig is enough.
These days I usually carry my Ashdown LB30 (30W tube head) and an older EA 2-10. They get the job done plenty. I do put the whole thing on a mixer stand to the 10s are hitting me in the shoulder blades while I'm standing directly in front of it. But even 15 ft away when I'm singing I hear it fine.
Watts alone are only a small piece of the pie that will determine if your rig is enough.