Hydrogen energy storage is two black boxes chained together: an electrolyzer turns grid electricity into hydrogen, a tank holds it, and a fuel cell turns it back into electricity when you need it. Because energy makes that round trip through two separate conversions instead of one, round-trip efficiency is the electrolyzer's efficiency multiplied by the fuel cell's efficiency — two lossy steps stacked, not added, so even an efficient stack returns only a bit over a third to two-fifths of what went in. What that trade buys back is a tank that barely leaks: its leak rate is small enough that its half-life runs in the hundreds of days, letting hydrogen bridge storage gaps measured in seasons rather than hours. Set the tank capacity, power rating, electrolyzer efficiency, fuel-cell efficiency, leak rate, and hold time, then watch tank level, power, round-trip efficiency, and retention toward half-life play out live — with stored energy converting to a hydrogen mass at 33.3 kWh per kilogram. It all runs on your device.
You are in the Physics lab.
Energy in during electrolysis, energy out during fuel-cell discharge — and where the rest goes.
What the schematic, the energy-split bar and the two plots are telling you — and how the store balances round-trip loss against long-duration retention.
Electricity from the grid flows into the electrolyzer during charging, some of it lost as heat, the hydrogen produced flows into the tank, and on discharge it flows back out through the fuel cell — with another loss — before reaching the grid. The gauge beside the schematic tracks live power against the store's rated power, positive while making hydrogen and negative while generating electricity from it.
The energy-split bar breaks the round trip into three slices: the electrolyzer loss and fuel-cell loss that both leave as heat, and what's left as energy actually delivered back to the grid. Round-trip efficiency is delivered energy divided by energy in — the electrolyzer's efficiency times the fuel cell's efficiency — so raising either efficiency slider grows that delivered slice.
The tank-level plot tracks percent stored through the electrolysis, hold and fuel-cell phases of one cycle, including the slow bleed of the leak rate during the hold. The retention plot zooms in on that hold, tracking stored energy decaying toward the tank's half-life — the time to lose half of what's left — reported live alongside the current H₂ mass still in the tank.
Two lossy conversions bracketing an almost loss-free hold — power-to-gas, then gas-to-power, with months to spare in between.
Making hydrogen with an electrolyzer loses some input electricity as heat, and turning that hydrogen back into electricity with a fuel cell loses more energy as heat on the way out. Round-trip efficiency is the fraction of energy in that comes back out, and because it's two conversions in series, it's the electrolyzer's efficiency multiplied by the fuel cell's efficiency — not either one by itself. Two numbers under 100% multiplied together always shrink further than either alone, which is why even a high-efficiency stack only returns a bit over a third to two-fifths of what went in.
Once the hydrogen is made, the tank just sits there — no motor winding down, no rotor dragging against bearings and air. Its leak rate is small: a fraction of a percent of stored energy per day, so its half-life — the time to lose half of what's left — runs into the hundreds of days. That's the opposite trade a fast-spinning mechanical store makes, where energy stored as motion bleeds away within hours; hydrogen pays its price up front in a lower round-trip efficiency, then holds what's left for months at a time.
The tank's stored energy is tracked in kilowatt-hours, but hydrogen itself is a mass of gas, and the two are related by a fixed specific energy: about 33.3 kWh per kilogram. Dividing the stored energy by that number gives the H₂ mass currently in the tank; multiplying a mass back by 33.3 gives its energy content. The electrolyzer and fuel cell are treated as sealed black boxes here — only their efficiency percentages and this one conversion factor matter, not the electrochemistry, membranes or catalysts working inside either one.