> What does it mean in practice to have energy instead of money as currency?

Something like Bitcoin. When the progress in miners efficiency stalls any kWh of energy not used for something else will be used to make some amount of bitcoins. If you have energy you can make btc. If you have btc you can give your btc to someone in exchange for their energy so that they give you their energy, instead of using it to mine bitcoins themselves.

That just seems very wasteful when proof-of-stake works.

It's not really wasteful if you use excess energy for mining that you couldn't store because storage is very limited.

The goal is not efficiency. The goal to provide incentive to overbuild the renewables.

Or even good old centralised distributed systems that do it another 4 to 6 orders of magnitude more efficiently than PoS.

It sounds terrible when you approach it from the point of money. Of course you can do money more efficiently. But if you approach this form the side of energy it's a way to organically tie a value to any energy produced. Even the energy produced at times when production vastly exceeds the demand. And that's going to be most of the energy produced since we need to develop renewables capacity and can't really wait for the storage technologies that lag horribly so we can match the supply the demand.

This is a way to make all energy valuable and providing incentive to build renewables even when 90% of the energy they produce will find no traditional buyer.

Using crypto to consume stranded wind and solar is an elegant idea.

> But then does that turn into famine and mass-die-off

Probably extremely bloody revolt and collapse

Only if you assume people's major motivation is wanting what they don't have, as opposed to wanting a little more to survive. History shows the opposite.