Implementing calendar-based power reserve includes identifying a new calendar entry scheduled into a calendar application of a mobile communications device. The new calendar entry specifies a telephone call. Upon determining a priority indicator has been set for the new calendar entry in the calendar application, the calendar-based power reserve also includes directing a power management component of the mobile communications device to reserve an amount of power for the telephone call, and updating a battery power indicator on the mobile communications device to reflect a capacity of a battery of the mobile communications device. The capacity reflects a current amount of available power for the battery minus the amount of power to reserve.