  Future galaxy redshift surveys aim to measure cosmological quantities from
the galaxy power spectrum. A prime example is the detection of baryonic
acoustic oscillations (BAOs), providing a standard ruler to measure the dark
energy equation of state, w(z), to high precision. The strongest practical
limitation for these experiments is how quickly accurate redshifts can be
measured for sufficient galaxies to map the large-scale structure. A promising
strategy is to target emission-line (i.e. star-forming) galaxies at
high-redshift (z~0.5-2); not only is the space density of this population
increasing out to z~2, but also emission-lines provide an efficient method of
redshift determination. Motivated by the prospect of future dark energy surveys
targeting H-alpha emitters at near-infrared wavelengths (i.e. z>0.5), we use
the latest empirical data to model the evolution of the H-alpha luminosity
function out to z~2, and thus provide predictions for the abundance of H-alpha
emitters for practical limiting fluxes. We caution that the estimates presented
in this work must be tempered by an efficiency factor, epsilon, giving the
redshift success rate from these potential targets. For a range of practical
efficiencies and limiting fluxes, we provide an estimate of nP_{0.2}, where n
is the 3D galaxy number density and P_{0.2} is the galaxy power spectrum
evaluated at k=0.2h/Mpc. Ideal surveys must provide nP_{0.2}>1 in order to
balance shot-noise and cosmic variance errors. We show that a realistic
emission-line survey (epsilon=0.5) could achieve nP_{0.2}=1 out to z~1.5 with a
limiting flux of 10^{-16} erg/s/cm^{-2}. If the limiting flux is a factor 5
brighter, then this goal can only be achieved out to z~0.5, highlighting the
importance of survey depth and efficiency in cosmological redshift surveys.
