Water electrolysis is not a simple process. It's carried out in two-compartment cell separated by a membrane. H2 and O2 are formed in the ratio 2:1. In order to keep the same pressure in cathodic and anodic compartments, you can use a storage tank for O2 twice smaller by volume than for H2. The maximum pressure in the tanks could be very high, the cost of the electrolyzer will limit the upper pressure.

The standard Gibbs energy for water splitting is deltaGo = 1.24 V for 1 atm O2 and H2. With an increase of pressure P in the storage tanks, deltaG = 1.24 + 0.12*logP. Thus, at pressure 10 atm, you will need to apply a potential at least 0.12 V higher.

I can only agree with Yurii V Geletii and would like to add. Should the electrolysis power be defined and thus the hydrogen output pressure be known. Can this electrolysis output pressure be the maximum hydrogen storage pressure. If the electrolysis output pressure and the internal hydrogen storage pressure are equal, the electrolysis operation must or will be switched off.