The power reserve of a watch movement refers to the amount of time a fully wound or charged watch can operate without needing to be wound or worn. It's an essential consideration for both manual and automatic movements. A longer power reserve indicates that the watch can continue functioning for an extended duration, even if it's not worn or wound daily. For automatic movements, the power reserve is determined by the tension stored in the mainspring after being wound. This feature is particularly useful for wearers who rotate between multiple watches or occasionally forget to wind their timepieces.