The Engine equipped on your ship will give you a certain amount of energy available for the ship to use.
Weapons, Armors will consume energy when installed on your ships, the amount of energy depends on the quatity and quality of the weapons and armors installed.
If the max energy is exceeded during assembling, you won't be able to assemble this ship. For maximum efficiency and firepower, it's often best to aim to add weapons and components to your designs in a way that gets the energy as close to its maximum as possible.