The suggestions about analyzing the fecal pellet are on the money. Valuable info will be gleaned by looking for seeds, hair, fur, bones, fiber, etc. It is not large enough for raccoon. Are you finding them one at a time as pictured? As opposed to a small pile or trail? It is mindful of an "owl pellet", but I can't imagine an owl roosting on a porch. Maybe another raptor. Are there rafters overtop of the droppings to serve as a perch? Keep looking at different times of night. The sensor/camera idea is very good.
Bat droppings are very uneven and shiny; not smooth and rounded, but there is usually more. The size you describe is too large for bats though.