A Sustained Thermal Power Envelope may be generated by monitoring a circuitry-level temperature in a portable computing device, monitoring a skin temperature of the portable computing device, monitoring an ambient temperature, operating the portable computing device during multiple time periods during which a circuitry-level temperature remains substantially constant, determining an average skin temperature during each time period, adjusting the skin temperature data by subtracting the ambient temperature, and generating data pairs of adjusted skin temperature and the power consumed during the time period.