BTU questions

Im in the process of building my trailered cooker.
Half charcoal/wood smoker half propane.
My question is this.
What sorta BTU's do I need?
Im useing a 250 gallon tank to make my cooker.
Been thinking of cutting it in half then in half again to make a 120ish sized cooker. Make sense?

Nothing?
What sorta BTU's is a 3 burner propane grill?

Thanks
Been thinking about one side for propane for fast little cooks and still be able to wood/coal the whole thing for large cooks and some bit of smoking.