> have a big powerbank in every basement and you are done.

Especially in the case of a flooding, Fukushima style.

Flooding destroys alsl batteries obviously, but the slightly important difference to Fukoshima is, a saltwater batterie does not contaminate the area.

Besides that most areas aren't in the risk of flooding, there is litte preventing you to water proof your batteries to the point that submerging them into water has no ill effects. The housing of the batteries usually is water-proof anyway. Just look at all the Teslas driving through deep water. Oil tanks for heating are a huge problem with flooding though, as they tend to leak oil into the environment.

This is right. For grid connected storage, two things matter: 10-year total cost, and durability. At least a 30-year design life. (Well, reliability matters too: that 's implicit.)

Volume and mass are just whatever they are. They don't matter.