Looking at the top left corner of your screen, you'll notice this little doodad.
This is your energy bar. Indicated by up to 5 Green batteries (Red when used), by default you have 2 energy tanks, but you can increase the size and regeneration rate via augmentations.





Identified ingame as "Sarif Series 8 Energy Converter", this is easily seen in the aug menu. i wouldn't reccomend buying any right away, but if you have any spare praxis feel free to invest in it.




Here's a graph depicting the upgrades, i'd highly reccomend maxxing out the bar first, but if you want to stick with 2 bars, max out the recharge. i got both.

Now for the part where i explain energy regeneration.
The normal regeneration kicks in after 10 seconds and restores your energy in 30 seconds. This can be upgraded. and it really helps.
However, it will not restore a completely depleted energy cell, besides the 1st one, it can still restore one if it is partially depleted, For example, picking up and throwing a heavy object with it's respectable aug will deplete a low amount of energy, and will regenerate. Punching through a wall will deplete a full energy cell, and it will not regenerate (Unless it's the first Cell.)
How do you regain more than one energy cell? With these.
The Proenergy bar Restores 1 Cell.
The Proenergy pack restores 2 Cells.
The Proenergy jar restores 3 Cells.
So you'd want to carry around as many jars as possible right? Riiiight?

Wrong. Bars are the way to go. maybe packs too if you want, but never jars.
Why not? this is why.
FOUR INVENTORY SPACES. FOR THREE ENERGY CELLS. And the worst thing is, they don't stack. If you do stumble upon one, use it immediately, never carry it around.




On the other Hand, Bars restore more energy in a fraction of the inventory space, they stack up to five, fit in one space, and restore 1 Cell for each bar, take the amount of space that the jar takes up and fill it with bars, and you have Twenty, you heard me right. TWENTY Mobile Energy cells.















Packs are the odd one out here, they're just as common as bars, take up 2 slots, stack up to 3, and restore 2 cells, if you have the space, grab one, it's worth it.

Next is ways to conserve energy, first off, GET THE REFLEX BOOSTER AUG FOR TAKEDOWNS.
Multiple Takedowns with one Cell. and it looks awesome.




Now the gas Grenade.
Want to knock down a large group of enemies without using energy? get a gas grenade. knocks them unconcious.
Minor spoiler:
I find it especially useful for crowd control after getting locked out of Zhao's Panic room
I also use it for taking out all the enemies in one door while waiting for the funicular in picus
Also, keep in mind that although there may be several paths that require energy, use as little as possible, it is tempting to break that one middle wall, but break the last one instead.
Other than that, there isn't much else for conserving energy, just don't be careless with it.

Congratulations! you now know how to use energy correctly.
Remember, always use bars.
And remember, Gas Grenades save you lots of takedown energy.



Images are courtesy of deusex.wikia.com and my screenshots.