Pretty much the Energy bar would be a combination of 2 different factors: Stamina and Fatigue.
Stamina would be dependent only on food/water/fat levels. And Fatigue would be dependent on work/time and could be relieved by camp fires and housing.
Therefore, if your food/water are full and you are of a healthy body weight for your height than your Stamina regen should be somewhat fast (aka the stamina bar should fill in no more than 15 seconds at 100% food/water)
Now, how Fatigue is factored in is how much stamina you can regen. So the more work you do, the less stamina you can regen quickly (has no effect on how fast stam is regened, just how much). If you work intensely for an hour or 2, say logging or terraforming you might not be able to sprint at all until you rest next to a fire or in a house to lower your fatigue limits.
How this can be visualized? Stamina would be the standard green bar in the Energy bar right now. Fatigue could be a Red/Purple colored section moving from right to left on the same energy bar depending on how much fatigue you are suffering. If you have no fatigue then there is no Red/Purple in the energy bar. If you are at 100% fatigue it would reach to 50% of the energy bar from the right to left. You would still be able to regen stamina quickly to 50% however, it wouldn't be enough to sprint. Although you could run quite far depending on your skills.