The first I heard of the new energy feature being implemented into Dark Legends was in this review of the game at massively.joystick.com. Here is the sentence from that article: "Spacetime used the energy mechanic from social games and improved upon it to give it a little more depth and purpose in the game."
Link to full article here

After reading that I remembered the video of Cinco playing at MWC.
Here is the video

Here are cropped screenshots of the video at 41 seconds and at 51 seconds:


Now I hope that with all of the above information we can all safely assume that there will be some sort of energy in Dark Legends. And we can say that the timer is at least 3m 38s long and that there is a max amount of 25 energy that one can have at any given time. Also I believe it is safe to assume one can buy more energy due to the presence of a plus next to the bar.

What I do not know is how long the timer is, but my guess from experience with games like these would be 10-20 minutes. I also don't know what the energy is used for. Some of my theories as to what it could be used for are as follows (in no particular order):

1. Cost to run a dungeon - the energy is used to gain entrance to a dungeon, pretty simple... this would be a game-killer/pain in the butt
2. Special moves/Power moves/Enrage - the energy is used for certain skills that are simply too powerful to be used on a regular basis or used to "power up"...dumb idea, but would be preferred over theory 1
3. Mini-games - the energy is used to participate in mini-games that take place during cutscenes... I got this idea from the tentonhammer.com article where it says this: "active vignettes (actual cutscenes – rare in a mobile game - where players perform quick actions on an unlock timer such as glamming a target or stalking a neighborhood)"... sounds like mini-games to me
4. Crafting - energy is used to craft items. Items still drop off bosses, quests, and chests; other than that all crafting (crafting materials drop from monsters, bosses, and chests as well)... this would make getting gear sluggish, but it wouldn't wreck the game as much as theory 1
5. Farming - the energy is used to plant and harvest crops... for lols

In all seriousness there are infinite possibilities as to what the energy will be used for in Dark Legends because of the quote I have up top that says that they improved upon the energy mechanic. And how the energy will provide depth and purpose is beyond me, because in my opinion it will only add annoyance.

Hopefully the energy is used for theory 3 as that wouldn't hinder leveling or farming (for vanities... etc.) while theory 1 would be the worst because it would limit pretty much everything. Theory 1 would destroy power-leveling pretty much totally because people wouldn't want to waste their energy on a dungeon lower level than them. Also it would make it hard to be an Ambassador for Dark Legends unlike the other two titles because you could only help out 25 dungeon instances at a given time (unless it doesn't always cost 1 energy to run a dungeon)... unless STS programs it to give infinite energy to Ambassadors. The only plus of theory 1 that I could think of is people wouldn't want to leave the instance because they invested something into it... but it is kind of complicated here too because if the person gets kicked for a dumb reason after they enter they just used energy for nothing.

The more I think about it the more I'd like to think that theory 1 or 4 can't be possible as it would totally change that game from it's predecessors. But theories 1 and 4 also seem like the obvious answers because other energy based MMO's use them (Spiral Knights and Gunshine). If energy is used for theory 3 which I'm gonna say is the best and most likely theory of my theory list, then the timer can be way longer than 10-20 minutes because they are just mini-games. To have to do them as often as 10 minutes would be tedious and would get old fast.

One more thing I'd like to add is a stress that energy adds to the game. If your a serious gamer, like me, you want to make use of all available resources. Well when energy is a part of a game and has a cap I feel the pressure to get on before that energy caps out (at 25 energy in the case of this game). This, even though it's silly, would make me feel the need to get on and play every x hours to get full use of the energy timer. It's that little stress that really just ruins any game for me, I play games to not have any stress.

Well there it is, I tried to keep my total and utter hatred for energy in f2p games at a minimum

Note: All text at the top (colored in navy) is backed by credible sources, all text colored brown is my opinion.