  The standard deviations of the energy and Poynting power densities for an
isotropic random field near a perfectly conducting planar boundary are
characterized, based on quartic plane-wave expansions. For normal and
transverse components, different rates of decay exist as a function of
electrical distance from the boundary. At large distances, the envelopes for
the power are more strongly damped than for the energy, both showing inverse
power law decay. The decay for the standard deviation is generally one order
faster than for the corresponding mean. For the normally directed power flux,
its standard deviation near the boundary increases linearly with distance. The
relative uncertainty of the scalar power is much smaller than for the Poynting
power. Poynting's theorem for standard deviations is obtained and demonstrates
larger standard deviations of the energy imbalance and power flux than their
mean values.
