Energy

Why Hydrogen Storage Wins on Duration and Loses on Round-Trip

By Lucian — builder & engineer, LK Forge

Hydrogen energy storage converts electricity to hydrogen and back through two lossy steps, but the tank in between barely leaks. We swept the Hydrogen Energy Storage lab's model across hold time and leak rate for a 1,000 kWh, 200 kW plant to measure exactly how little the round trip changes over months of storage, and how much it gives up to get there.

 ·  5 min read  ·  measured from the round-trip efficiency benchmark

138.63 days
stored-hydrogen half-life at a 0.5%/day leak rate
38.5%
round-trip efficiency with no hold at all (70% electrolyzer × 55% fuel cell)
24.55%
round-trip efficiency after a 90-day hold at the same leak rate
30.03 kg
hydrogen mass stored in the 1,000 kWh tank

Two conversions set the ceiling; the tank barely moves it

A hydrogen storage system stores energy as a chemical fuel, not as motion or a charged electrode. Electricity runs an electrolyzer to split water and store the hydrogen; a fuel cell later recombines that hydrogen with oxygen to make electricity again. Both conversions carry real losses that apply once each direction, no matter how long the hydrogen sits in between. The tank itself only loses a small, slow leak over time — so unlike a flywheel's exponential bearing drag or a battery's self-discharge, the round trip here is dominated by the two conversion steps, not by how long you wait.

round-trip % = electrolyzer efficiency × exp(−leak rate × hold days) × fuel-cell efficiency

That's why the hold-day sweep below barely moves: the electrolyzer and fuel-cell efficiencies are fixed by the hardware, and at a 0.5%/day leak the exponential retention term stays close to 1 for weeks at a time.

Round-trip efficiency and retained energy barely fall over months

Round-trip efficiency (amber) and retained energy (blue), plotted against hold days for the 1,000 kWh plant at a 0.5%/day leak rate. Retained energy decays exponentially but slowly; round-trip efficiency tracks it closely, offset down by the fixed 70% × 55% conversion penalty.

0% 25% 50% 75% 100% 0306090 hold time (days) percent
retained energy round-trip efficiency

The exact numbers

Every row below is a direct sample from the lab's model: a 1,000 kWh, 200 kW plant, 70% electrolyzer, 55% fuel cell, 0.5%/day leak rate, for the hold-day sweep.

Hold time (days) Round-trip Retained energy
0 38.5% 100%
1 38.31% 99.5%
7 37.18% 96.56%
14 35.9% 93.24%
30 33.14% 86.07%
60 28.52% 74.08%
90 24.55% 63.76%

And the separate leak-rate sweep, for the same plant, at zero hold:

Leak rate (%/day) Half-life
0.1 693.15 days
0.5 138.63 days
1 69.31 days
3 23.1 days
5 13.86 days

And a few electrolyzer / fuel-cell efficiency pairs, at zero hold and zero leak time:

Electrolyzer Fuel cell Round-trip
70% 55% 38.5%
80% 60% 48%
60% 45% 27%

Plant: 1,000 kWh capacity, 200 kW power, 30.03 kg hydrogen at full charge, 70% electrolyzer efficiency, 55% fuel-cell efficiency. Generated 2026-09-11.

Try it yourself

Open the lab, set your own tank capacity, power, electrolyzer and fuel-cell efficiency, hold time, and leak rate, and watch round-trip efficiency and retained energy move in real time.

Open the Hydrogen Energy Storage lab →
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Common questions

What is round-trip efficiency for a hydrogen energy storage system?

Round-trip efficiency is the fraction of the electricity used to make hydrogen that you get back out as electricity later. It comes from three factors multiplied together: electrolyzer efficiency (turning electricity into stored hydrogen), storage retention (how much of that hydrogen survives the hold between charge and discharge, given a slow leak), and fuel-cell efficiency (turning the retained hydrogen back into electricity). For this 1,000 kWh plant at a 70% electrolyzer and 55% fuel cell, charging and discharging immediately round-trips 38.5% of the energy; two energy-conversion steps each carrying real losses is why that number starts well below either individual efficiency, even before any hold time is added.

Why does converting electricity to hydrogen and back lose so much energy?

Because the round trip goes through two separate conversions, and each one keeps only a fraction of what it's given. The electrolyzer keeps 70% of the electricity it's fed (as chemical energy in hydrogen), and the fuel cell keeps 55% of the hydrogen energy it's fed (as electricity). Multiplying 70% by 55% already caps the best-case round-trip at 38.5% before the tank has lost a single percent to leakage — unlike a battery, which stores and returns energy through one conversion step, not two.

Why does hydrogen storage hold its energy so much longer than a flywheel or battery?

Because the loss mechanism is a slow leak rate on a stored chemical fuel, not continuous mechanical drag or self-discharge on live circuitry. At a 0.5%/day leak, the stored hydrogen's half-life is 138.63 days — compare a flywheel spinning at a 30%/h standby rate, whose stored energy has a half-life of only about 2.31 hours. That is the tradeoff in one comparison: the flywheel round-trips about 84.64% of its energy if used immediately, far above hydrogen's 38.5%, but it cannot hold that energy for more than a few hours before most of it is gone. Hydrogen gives up round-trip efficiency for duration — over 90 days at this plant's 0.5%/day leak rate, retained energy only falls to 63.76% and round-trip efficiency to 24.55%, a loss of just 13.95 percentage points from a three-month wait.