What is the 247 TWh number referring too?

Germany uses a little over 500 TWh of electricity per year.

Germany has a little more than 20 GWh of grid storage.

It's referring to the possible storage capacity, probably the potential to store hydrogen in underground solution mined salt caverns.

I've heard much higher figures, in the PWh for Europe as a whole.

Storage of energy as heat at 600 C has much larger potential storage capacity; see standardthermal.com