Based on X-ray (1-8 ${AA}$) flux data for 1972-1995 the integral spectra of solar flare energy were computed. It has been shown that the spectral index $eta$ of the integral energy spectrum (IES) vanes systematically with the 11-year cycle phase. The interval of $eta$-variations (0.47 <$eta$<1) is characteristic of UV-Cet stars. The maximum energy of the X-ray flares does not exceed $10^{32}$ erg.