Abstract: A non-intrusive thermal power monitor and method for determining the amount of sensible heat withdrawn from or added to a fluid stream flowing in a conduit by an unknown source includes a thermal power transfer device that supplies or removes a known amount of heat energy to the fluid in a conduit. First and second temperature sensors sense the temperature of the fluid stream across the thermal power transfer device, and this information is used to determine a heat capacity rate of the fluid in the fluid stream. Additional non-intrusive temperature sensors sense the temperature of the fluid stream as it passes through and across a thermal power sink or heat source and provides a temperature differential. These temperatures are processed in a control circuit, and the thermal power added to or extracted from the fluid stream is determined in the control circuit by multiplying the measured temperature differential by the heat capacity rate of the fluid stream.