  Using the standard one-zone synchrotron self-Compton (SSC) model and the
Markov chain Monte Carlo (MCMC) technique, we systematically analyze the
quasi-simultaneous multi-wavelength (MWL) spectral energy distributions (SEDs)
of Mrk 421 and Mrk 501 during states of relatively low activity. With this
model in place, a semi-analytical method is developed to examine the
uncertainty in jet power estimation caused by degeneracy of the radiative
models. The semi-analytical method, in combination with the MCMC technique,
allows us to explore the jet properties over a wide range of the variability
timescale. Our results seem to support: (1) In both Mrk 421 and Mrk 501 the
jets are powered by rapidly rotating black holes (BHs). The BH spin in Mrk 501
could be lower than that in Mrk 421 or possibility they are equal under the
assumption of Blandford-Znajek mechanism. (2)The energy losses, which could be
used to form the large-scale radio structure, are important for reconciling the
differences of the kinetic power derived from the observations of the
large-scale structure and the SED fitting results. Moreover, the jet energy
losses in the propagation are more significant for Mrk 501 than for Mrk 421.
