Energy mass storage, or infinite storage

I'm not sure I implied that. In fact, I think it's just the opposite. A small energy storage creates the challenge of getting energy just right. Going over, as you mentioned, creates inefficiency that could be exploited. Going under, as I'm implying, will create consequences in a very short time. Either way is pretty bad, but the consequences are much more acute and magnified when compared to metal. The idea of an energy shortfall opens up an Achilles's heel for any sized base, which is great for invasions and sabotage. That wouldn't be possible if energy storage was big enough for a rainy-day fund.

The ideal is to get exactly the amount of energy that's needed. Realistically, it's a good idea to keep a surplus. This could be for defenses, commander abilities, or maybe because you don't want one lost generator to cause a brownout. The excess energy isn't necessarily wasted, because the safety net is more important to have.

I agree. Storage is important or late game would seem a bit crap.

I could see a little convoy of ships cruising to the next solar system with your com, an initial invasion force, and a stockpile of metal and energy (or maybe a landable powerplant) to jumpstart your new base...

I disagree with this though. a small energy storage just encourages you to build more pgens (in supcom). As I’ve argue before, in a rate-based economic model, excessive energy acts practically the same as its storage (in supcom at least). Having +200 energy was as good as having a whole bunch of energy in storage. Hence why storage in supcom was only used for exotic purposes (or until you needed a single storage for overcharge).

The difference is that if you can store energy without any cost you can save up energy for a teleport and perform an optimal teleport without stalling or having to pause other construction.
In SupCom though energy storages were expensive so it was more costeffective to make excess power for emergency needs rather than storages.
What should the role of energy storages be in PA?
It should allow the player to save up energy for tech switches, energy expensive upgrades, energy expensive teleports. Then it storage needs to be cheap.
It should create a buffer for varying energy sources such as wind generators and reclaiming trees. Then storage can be moderate in cost.
It should allow the player to fire the Dgun successionally or allow energy heavy weapons to fire. Then storage can be expensive.