Energy

Energy in the Sims 1
If your Sim runs out of Energy, he/she will collapse of exhaustion, and falling asleep on the floor will not do the Comfort meter any favors. The simple way of sorting this out is sitting down or sleeping. If a Sim does make it to bed before they collapse, he or she will automatically sleep until at least 6 am. Otherwise, he/she will sleep until the Energy bar is back at 100%. Sending a Sim to work or school with low energy is not good, and although they won’t collapse from exhaustion away from home, they can do as soon as they return – and then there’ll just be an unconscious Sim littering the pavement. If a Sim needs a quick boost of energy, they can drink coffee or espresso (although children can’t do that).

When you go out of the house to places such as Studio Town and Old Town, your Sim's Energy bar will naturally decrease. Your Sim can buy an espresso from the espresso bar, but really, the best thing to do is simply go home when your Sims gets too exhausted.

Pets

Pets lose energy quite quickly if they are running around, and so need to sleep about twice a day. It is therefore essential that you provide them with some baskets or doghouses for them to sleep in.

Energy in the sims 2
In the sims 2 the only real way to replenish your energy is to sleep. your energy slowly decreases when awake but decreases alot when working out. Also it decreases more than little when at work. If you take a nap then it very slowly increases.