Ecosystem is an example of system.
Explanation
The best example of a system is the ecosystem.
The energy flow in the ecosystem starts from the sun which is the primary source of producing energy.
The sun provides energy to the plants and which will transfer energy to the animals or humans consuming them.
The energy is further transferred between the organisms by consumption of other organisms.
Even the decayed organisms are converted to fuel energy and the waste or unused energy is released as heat in the atmosphere.
Thus in the ecosystem, the law of conservation of energy is maintained as the energy transferred by the Sun to other organisms can be again converted to fossil energy once they have decayed.
