  Quintessence can cluster only on horizon scales. What is the effect on the
observed matter distribution? To answer this, we need a relativistic approach
that goes beyond the standard Newtonian calculation and deals properly with
large scales. Such an approach has recently been developed for the case when
dark energy is vacuum energy, which does not cluster at all. We extend this
relativistic analysis to deal with dynamical dark energy. Using three
quintessence potentials as examples, we compute the angular power spectrum for
the case of an HI intensity map survey. Compared to the concordance model with
the same small-scale power at z=0, quintessence boosts the angular power by up
to ~15% at high redshifts, while power in the two models converges at low
redshifts. The difference is mainly due to the background evolution, driven
mostly by the normalization of the power spectrum today. The dark energy
perturbations make only a small contribution on the largest scales, and a
negligible contribution on smaller scales. Ironically, the dark energy
perturbations remove the false boost of large-scale power that arises if we
impose the (unphysical) assumption that the dark energy is smooth.
