  We analyse a compressible Poiseuille flow of ideal gas in a plane channel. We
provide the form of internal energy U for a non-equilibrium stationary state
(NESS) that includes viscous dissipation and pressure work. We demonstrate that
U depends strongly on the ratio {\Delta}p/p_0, where {\Delta}p is the pressure
difference between inlet and outlet and p_0 is the outlet's pressure. In
addition, U depends on two other variables: the channel aspect ratio and the
parameter equivalent to Reynolds number. The stored internal energy,
{\Delta}U=U-U0, is small compared to the internal energy U0 of the equilibrium
state (ES) for a moderate range of values of {\Delta}p/p_0. However, {\Delta}U
can become large for big {\Delta}p or close to vacuum conditions at the outlet
(p_0~0 Pa).
