Assumptions:

Based on 12,000 kWh per person per year, the daily requirement is 32,877 MWh for 1 million people.

Hydrogen generation efficiency of 80 pct.

Electricity regeneration efficiency of 50 pct.
Another example is providing electrical power to a city with a population of 1million people. Two examples are provided: (1) where 100 pct of the power is provide from stored renewable energy (solar or wind energy) and (2) where 50 pct of the power is provided form stored renewable energy.
Example 1: 100 pct of power from stored renewable energy:

Chamber size: 20 ft diameter, 2000 ft deep

Pressurized to 5,000 psi

Number of chambers required  3.3
Example 2: 50 pct of power from stored renewable energy:

Chamber size: 20 ft diameter, 2000 ft deep

Pressurized to 5,000 psi

Number of chambers required 1.6