Board Thread:Bloons Wiki and BTD News/@comment-5056929-20130809071543/@comment-3153244-20130815072150

Energy is used to attack, move around and to upgrade (Tier 2 upgrades max!). However, there aren't going to be any attacks that use up extra energy. Powerful attacks are just going to require higher energy for a greater chance of upgrade success. Upgrading will cost you 3 energy, although successfully upgrading also gives you more max energy. ;)