The capacity that allows players to attach parts to their robots. Better weapons and gadgets require more energy because they have more power, so the body with better tier (better amount of energy capacity) will have more room for them.
Energy is a stat of the body that indicate the ability to assemble weapons and gadgets on that body.
Energy symbol in Inventory
Depending on the type and level of the body, the body has a permanent amount of energy. Meanwhile, each weapon or gadget has a power stat which diminish the energy of the body when it's assembled.
When assembling weapons and gadgets into the body, make sure the total power is less than or equal to the energy of the body; otherwise you will not be able to complete the desired robot.
For instance: The Battleship body has 15 energies, you can't assemble a robot if installed weapon and gadget have total power greater than 15
Battleship body after assembling weapon