Energy system aka TECS stands for Total Energy control System. Initially, it was implemented in manned aircrafts. The desired outcome directed this idea toward unmanned aircrafts. Simple underlying concept sheaf behind this system includes the balance of gravitational potential energy and kinetic energy.
Gravitational potential Energy = mass x gravity x height
Kinetic Energy = ½ x mass x speed2
The object holds gravitational potential energy due to its height. As per the equation, Gravitational potential energy increases as per the increment of height. In other words, for increasing the height of object, it requires more energy. In the case of drone, to increase the height requires more energy, which means more throttle required.
Apart of it, the object holds kinetic energy due to its speed. And the energy is proportional to the square of speed. This means, to increase the speed of drone, requires more energy.
The total energy of the drone is the addition of gravitational potential and kinetic energy. As we already know, four forces works on drones in air. Among of them, Frictional force, which is named as drag, reduces the total energy of drone. To maintain the altitude and speed of drone needs to adjust throttle for thrust variation. During flight, TECS calculates total required energy and adjust the throttle to maintain desired speed and altitude. As well as TECS maintain the balance between kinetic and potential energy by adjusting the required pitch angle.