Various embodiments of methods and systems for thermal energy management in a portable computing device (“PCD”) based on power level calculations are disclosed. An exemplary method includes tracking instantaneous operating temperatures and active power supply levels to one or more components. With an estimate or measurement of ambient temperature, the instantaneous operating temperature values and active power supply level values can be used to calculate an instantaneous thermal resistance value. In the event that thermal energy generation should be managed, a target operating temperature may be used with the ambient temperature and the instantaneous thermal resistance value to solve for an optimum power supply level. The active power supply level may then be adjusted based on the calculated optimum power supply level.