Storage is possibly the most pernicious bottleneck for hydrogen use, because of hydrogen’s very low energy density (see last paragraph below). We use a 500 gallon propane tank for storage, at 200 psi maximum pressure. A project at Humboldt University , CA uses this same method.
We were told the tank must be new, because a used tank might have residual gases which could poison the fuel cell (but why would thorough purging with nitrogen not eliminate this?). The tank may have a limited lifespan, due to the reactive nature of hydrogen and consequent risk of corrosion (see Hydrogen Safety). Presumably the reactivity of hydrogen is increased by moisture in the tank (since this would lead to loose H+ ions in aqueous solution, ie an acid). Most electrolyzers have a built-in dryer to ensure dry hydrogen.
While higher pressures (eg 2500 psi, as in commercial K-cylinders) would have vastly increased our storage capacity, the cost and complications of adding a pressure pump kept us to 200 psi. Also, an off-the-shelf propane tank is rated to only 215 psi (we do not know whether it can be rated to a higher pressure). Conveniently for our system, our electrolyzers put out hydrogen at 200 psi.
We chose a 500 gallon tank size for three reasons. First, our current electrolyzers are small and therefore cannot fill a larger tank in any reasonable time. Second, we have only a 1 kW fuel cell. Third, we figure we can add another 500 gallons of capacity easily. In short, we built our system on a relatively small scale in all parts, to control price and because we were experimenting.
That said, bigger is better when it comes to storing hydrogen (See Useful Equations to follow this calculation): In a tank filled to 200 psi, you have 160 psi (11 atmospheres) of usable gas, since the fuel cell has a minimum 40 psi feed. Thus our 500 gallon tank at 200 psi stores 5500 gallons ≈21,000 liters (STP) of usable H2. At 3.5 Wh/liter, this is about 73 kilowatt-hours of power. Given a fuel cell electrical efficiency of 40%, the effective stored electric power (measured “at the plug”) of our 500 gallon tank is about 29 kWh. This is equivalent to 3½ commercial K-cylinders of hydrogen, or a bank of 30 L-16 batteries. If your electrolyzer is big enough, a 1000 gallon tank is desirable. Its footprint is not much greater and it doubles your energy reserve.