In an accommodation pump gas is transferred from one point to another at the same temperature without the application of any external mechanical or electrical forces to the gas-only temperature gradients are used. If there is no continuous flow a static and unchanging pressure difference develops between the two points, requiring no additional input of energy to the gas. Little attention has been paid to the energy transfer, i.e., the number of calories per mole of gas pumped, in this process. A simple model is presented in which this energy transfer is analytically calculated for a single-stage Pyrex accommodation pump, upper temperature room (295 K), lower temperature liquid nitrogen (77.4), gas helium, compression ratio 1.2, and pressure in the free molecular range. Under these conditions the physical adsorption of helium on pump surfaces is quantitatively negligible. The model calculates the energy difference between a true accommodation pump with atomically rough (leached) and smooth surfaces and a nonpump with exactly the same geometrical dimensions, but with all surfaces rough. The energy differences are assigned to accommodation pumping. It is found that an energy in the range of 300 calories per mole of gas pumped is required, independent of the overall size of the pump, but becoming smaller as the cold volume is reduced relative to the warm volume. The results are extended to multistage pumps, which are ponderous to calculate analytically, but which bear a simple relationship to a single-stage pump.