A pumped-hydro plant is a black box you pump uphill and generate back down, and it never gives back exactly what you put in: its round-trip efficiency — energy delivered over energy drawn — falls short of 100% because the pump, the water in the tunnel, and the turbine each lose some of it along the way. Push more flow through the same tunnel and friction head loss rises with the square of that flow, so fast pumping or generating wastes far more energy than a gentle rate. Hold the reservoir full and evaporation and seepage quietly drain a little of it away even while idle. And how much energy the plant can hold at all comes down to head times reservoir volume, which together with the flow rate also set how long a full reservoir can sustain generation. Set the head, reservoir volume, pump flow, generation flow, machine efficiency, and hold time, then watch reservoir level, power, and round-trip efficiency play out live. It all runs on your device.
You are in the Physics lab.
Energy in while pumping, energy out while generating — and where the rest goes.
What the schematic, the energy-split bar and the two plots are telling you — then what head, volume and flow do to capacity and duration.
Energy from the grid flows in while pumping, some of it is lost along the way, the rest is stored as potential energy in the upper reservoir, and on generation it flows back out through another loss before reaching the grid. The gauge beside the schematic tracks live power against the plant's rated power, positive while pumping and negative while generating.
The energy-split bar breaks the round trip into three slices: the pump loss, friction loss, and turbine loss that leave as heat and turbulence, and what's left as energy actually delivered to the grid. Round-trip efficiency is delivered energy divided by energy in — the higher the flow rate or the weaker the machine efficiency, the smaller that delivered slice gets.
The reservoir-level plot tracks percent of full volume through the pump, hold and generate phases of one cycle, including the slow bleed of evaporation and seepage during the hold. The efficiency curve tracks round-trip efficiency against flow rate — because friction head loss scales with flow squared, efficiency falls faster than flow rises, reported live here as round-trip and peak power.
A friction loss on every cubic meter that flows, an evaporation and seepage that never sleeps, and a capacity set by head times volume, not by anything inside the powerhouse.
Water moving through the tunnel or penstock dissipates energy to friction head loss that scales with the square of flow rate, on both the pumping and generation legs. Round-trip efficiency is energy delivered divided by energy in, and it drops whenever pump flow or generation flow rises or machine efficiency is worse — a fast, high-flow cycle wastes far more of the round trip to friction than a gentle one, even though it moves the same volume of water.
Even with no pumping and no generation running, an open reservoir slowly loses volume to evaporation off its surface and seepage through its bed and dam. It's usually a small fraction per day, but over a long hold time between pumping and generating, it measurably lowers the reservoir level generation starts from, on top of any friction loss already paid during pumping.
Stored energy is proportional to head times reservoir volume — the potential energy in a mass of water is its weight times how high it's raised, so a taller head needs less volume for the same capacity, and a shorter head needs more. Storage duration is volume divided by flow rate — how long a full reservoir can sustain generation at the chosen generation flow before it's drained, reported live here alongside peak power.