A post hole digger is one of the few implements where the wrong purchase is obvious within the first hour of use. Too small an auger and you spend the day widening holes by hand; too large a one on an underpowered tractor and the bit stalls in the first patch of clay. Neither failure shows up on a specification sheet.
The safer approach is to work backwards from three measurements you can take before spending anything: the finished diameter your posts, piles or solar legs require, the depth your drawings specify, and the hardest soil anywhere along the route. Then choose an auger at or just below the tractor’s comfortable limit, not at its maximum. A machine running at 80 percent of capacity finishes a campaign; one running at 100 percent finishes the first week and then visits the workshop.
Tractor Auger