You can get more energy simply by waiting! You receive one Energy point every two minutes and the maximum Energy you can receive this way is 100.

You can also get more Energy by collecting and merging the Energy lightning bolts that you can get as rewards from Energy Chests. Energy Chests can be received from watching ads or participating in events and getting rewards.

Lastly, you can fill up your Energy bar with 10 Gems, and each time you replenish it, the cost will rise.