The energy is always conserved for a system which is

The principle of energy conservation states that energy is neither created nor destroyed, only transformed [1]. For energy to be 'always conserved' within a specific system, that system must be isolated. An isolated system is defined as one that exchanges neither matter nor energy with its surroundings. In contrast, a closed (non-isolated) system allows energy to cross its boundaries, and an open system allows both matter and energy exchange. While the First Law of Thermodynamics applies to all systems by accounting for energy transfers, the total energy remains constant (conserved) only within the boundaries of an isolated system because there is no inflow or outflow to alter the total sum [1]. Therefore, energy is strictly conserved only for a system that is isolated from its environment.

Sources

1. [1] https://www.sciencedirect.com/topics/engineering/conservation-of-energy-principle