M
mysteryman
It is true that the thermal conductivity of copper is better than that of stainless steel. However, it is the boundary layer on each surface of the metal where most heat resistance occurs, and this masks the effect of the different metal. A declared rate of 20 kW is still 20kW regardless of the material used in manufacture. The relative volume of the coil is not important; the heat is inside the cylinder whether or not it is also inside the coil.