[philipkglass]

I think that you slipped some decimal places.

Propane has a thermal energy content of 13778 watt-hours per kilogram [1]. That's (25000 * 10^12) / 13778 = 1,814,486,863,115 kilograms, or 1.8 billion tons. 3.6 billion tons of propane if you recover electricity at 50% efficiency. That would make the 300 million ton underground storage equivalent to one month of global electricity demand. That's still a lot of storage, of course.

[1] https://en.wikipedia.org/wiki/Energy_density#In_chemical_rea...

[ajuc]

Thanks, should have double checked. I always mess up long and short scale when combining from different sources.