BigBen2k,
Here's another application of the "input + produced = output + destroyed + stored" thingie. When you turn on a pump with a manometer, there is an energy involved in pushing the liquid up the tube. Once it reaches a steady-state elevation, it is potential energy with no further energy input. After this point, all energy from the pump goes into pushing the fluid around the loop. Once you turn off the pump, the potential energy in the manometer tube settles out to equilibrium.
It's sort of like stretching a rubber band. It takes energy to stretch it (pump work) but once stretched, no further work occurs until the length of the rubber band changes. Sure, it takes a force to maintain the stretch, but this is equivalent to the discharge pressure from the pump. Remove that force (turn off the pump) and the energy stored in the rubber band (water column) is returned as it reaches its normal rest position.
Make sense?
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