The energy balance in the thermoelastic effect is analysed in the framework of the mechanically deformed ensemble of anharmonic oscillators. As the result, two aspects of mechanical loading - the macroscopic deformation of a sample, with corresponding addition to the energy balance (the mechanical work), and the vibration energy of oscillators change at the microscopic level, are separated. Between these two parts of the internal energy of the sample the exchange in the thermo-elastic effect takes place. The low temperature limit in which the vibration energy reduces to the energy of zero vibrations is also considered.