  Practical application of the harmonic summing technique in the power-spectrum
analysis for searching pulsars has exhibited the technique's effectiveness. In
this paper, theoretical verification of harmonic summing considering power's
noise-signal probability distribution is given. With the top-hat and the
modified von Mises pulse profile models, contours along which spectra total
power is expected to exceed the 3\,$\sigma$ detection threshold with 0.999
confidence corresponding to $m=1, 2, 4, 8, 16$, or 32 harmonics summed are
given with respect to the mean pulse amplitude and the pulse duty cycle.
Optimized numbers of harmonics summed relative to the duty cycles are given.
The routine presented builds a theoretical estimate of the minimum detectable
mean flux density, i.e. sensitivity, under the power-spectrum searching method.
