The Water Mill is an Industrial Craft 2 generator. Water Mills can generate EU either through the use of Water Buckets or Water Cells being placed inside their inventory, or by being placed in a body of water.
Internally, the Water Mill's tank will hold four buckets of water. Using their internal storage, Water Mills produce 1 EU/t. If placed in water, either still or flowing, the Mill will generate EU passively, at a rate of 0.01 EU/t per block of adjacent water. Assuming one block is used for cable, the maximum EU one Water Mill can generate is 0.25 EU/t.
The Water Mill's GUI features two slots. The top slot can be used for recharging tools and batteries. Water Buckets or Water Cells are placed in the bottom slot of the GUI.