Peltier AC Operating curve
Qc is the amount of heat that the device can pump from the hot side to the cold side.

Even with zero thermal load on the cold side it can only maintain ~42° deltaT.

With zero deltaT the device can pump 300W.

The graph dictates how much heat sinking you need to provide for the device to obtain a specific amount of cooling. For example if you want to maintain 20° cooling you can only support 160W of pumping.

Peltier devices have significant amounts of thermal conduction within the device itself from the hot to cold side that has to be supported in addition to the heat pumped from the cold side.

The heat sink not only needs to cool the heat pumped but also the losses of the Peltier device. There should be another graph giving information on the amount of electrical power input at that working point.