Users upon registering start off with a max of 200.
After some of it is used, it replenishes at a rate of 1 every 150 seconds (2,5 min.). You will get 24 energy every hour, so after 8 hours 20 minutes, you will have full energy if you are an amateur.
the Max energy can be increased by finding magic coins, which you can find by getting coins on any world. It gives you +1, +2, +10, +15 of max, with +10 and +15, a full recharge of energy, and +5 occasionally a full recharge of energy (possibly +30 will be back). Chris had removed the magic coins for energy because of people abusing the system by using bots like EE bot or Coin Collector, but since the update of February 7, 2015, you can again increase your energy with magic coins. There's also the chance you can get a wizard, fire wizard, witch, dark wizard or purple ghost (former), but it is much harder to find now. The grinch was a Christmas edition only magic coin smiley.
Previously was possible to earn max energy by leveling up.
There was a bug where you could get negative energy after you spent your energy. This only happens sometimes and if you play a world then come back to it, it will be normal again.