The swell and shrink phenomena make controlling the water level in steam generators extremely difficult, along with the flow errors. Therefore, quantitative evaluation of the swell and shrink is interesting and valuable in analyzing the nuclear steam generator and in the design of its controller. The power spectral density function (PSDF) distributes the power of the signal for frequencies. This excitatory input signal is assumed to be the band-pass white noise, which is more realistic than the ideal white noise. The integration of the PSDF over frequency is defined as energy. We derive the energy contribution of each dynamic mode using the PSDF. Its energy contribution can be calculated easily by arithmetic calculation.