The rate of flow of heat between two systems is measured in watts (joules per second). The formula for rate of heat flow is ∆Q/∆t = -K×A×∆T/x, where ∆Q/∆t is the rate of heat flow; -K is the thermal conductivity factor; A is the surface area; ∆T is the change in temperature and x is the thickness of the material (∆T/x is called the temperature gradient and is always negative because of the heat of flow always goes from more thermal energy to less).
Example
Assume there are two systems with the same mass and specific heat. System A has an average temperature of 500 kelvin and system B has an average temperature of 400 K. If thirty seconds after the systems are put in contact they both reach 450 K, then the average rate of heat flow is:
50 J/30 s, or rather 1.67 W.
(note: specific heat capacities = 1 J/(kg K) and masses = 1 kg)