Minecraft wiki entry for Miscellaneous, associated text: When the input turns on, the dropper pushes an item into the hopper, which is placed in the hopper's left slot. It takes 23 64-stackable items (or five 16-stackable items and three 64-stackable items, or six 16-stackable items) to produce power level 2, so if the 64-stackable item is pushed that isn't sufficient to increase the output power level, but if the 16-stackable item is pushed the output power level increases to 2. And if the non-stackable item is pushed, the output power level increases to 4.