Water-flow analogies are helpful in understanding electricity. For example, in the Lodge model, the constant DC voltage source (a battery) is represented by a U-tank with two water columns of the same cross-sectional area connected by a horizontal duct in which a pump is installed. The pump maintains the difference of the levels of the two water columns, which corresponds to the constant emf of a battery. In this article, the author discusses how to successfully map the U-tank to an electric circuit. This work will be beneficial to instructors who teach electricity because they can recognize the validity and the limitation of the liquid-flow analogy (that is, to which electric circuit the U-tank system is mapped successfully or not). It can be useful in forming a basis of analogy-based learning. (Contains 1 figure.)