The concept of energy only ever has any use in the context of the law of energy conservation. That law, however hypothetically true (as evidenced by practical unconstructability of perpetual motion devices), is unenforceable in any PHYSICAL system due to the demand of the system to be isolated, which is impossible because physics only studies OBSERVABLE phenomena, the act of observation being a particular of the act of interaction, therefore isolation-condition-breaching. In other words, the systems with holding law of energy conservation are unobservable, thus unphysical (meaning they cannot be the subject of a physical study) - or have to fully account for the presence of a HUMAN observer, his entire psysiology and psyche included, said human observer being an incomparably more complex system than any practically studied physical one.

Therefore, while convenient in principle, the law of energy conservation only ever works as an approximation with an unknowable-in-principle case-to-case margin of error, accounting for the influence of the human observer's presence on system in each particular case.

Fucking fight me.