Question about energy storage

I have done a few searches, and while I find energy storage mentioned in numerous places, I haven't actually been able to find any information about how much extra energy one gets a point.

So here are my questions:

#1. Each time you increase energy storage how much extra energy is your max increased by?

#2. Does ES also increase regeneration time?

#3. For players who have gone up to a high level: How much does energy storage matter to casters. Do you find yourself running out of mana often?

I am unsure if fast casting, for example is more useful than energy storage, or if better armor makes up for energy storage.

#1 3 energy per point
#2 no
#3 Depends on play style, spell choice and energy efficiency of the player.