  The Square Kilometre Array (SKA) will conduct the biggest spectroscopic
galaxy survey ever, by detecting the 21cm emission line of neutral hydrogen
(HI) from around a billion galaxies over 3/4 of the sky, out to a redshift of
z~2. This will allow the redshift-space matter power spectrum, and
corresponding dark energy observables, to be measured with unprecedented
precision. In this paper, we present an improved model of the HI galaxy number
counts and bias from semi-analytic simulations, and use it to calculate the
expected yield of HI galaxies from surveys with a variety of Phase 1 and 2 SKA
configurations. We illustrate the relative performance of the different surveys
by forecasting errors on the radial and transverse scales of the baryon
acoustic oscillation (BAO) feature, finding that the full "billion galaxy
survey" with SKA2 will deliver the largest dark energy figure of merit of any
current or future large-scale structure survey.
