We have carried out the largest (>3.5 x 10(5) Mpc(3), 26 deg(2)) H alpha narrow-band survey to date at z similar to 0.2 in the SA22, W2 and XMMLSS extragalactic fields. Our survey covers a large enough volume to overcome cosmic variance and to sample bright and rare Ha emitters up to an observed luminosity of similar to 10(42.4) erg s (1), equivalent to similar to 11M(circle dot) yr(-1). Using our sample of 220 sources brighter than > 10(41.4) erg s (1) (> 1M(circle dot) yr(-1)), we derive Ha luminosity functions, which are well described by a Schechter function with phi(star) = 10 (2.85 +/- 0.03) Mpc (3) and L-H alpha* = 10(41.71 +/- 0.02) erg s (1) (with a fixed faint end slope alpha = -1.35). We find that surveys probing smaller volumes (similar to 3 x 10(4) Mpc(3)) are heavily affected by cosmic variance, which can lead to errors of over 100 per cent in the characteristic density and luminosity of the Ha luminosity function. We derive a star formation rate density of rho(SFRD) = 0.0094 +/- 0.0008M(circle dot) yr(-1), in agreement with the redshift-dependent H alpha parametrization from Sobral et al. The two-point correlation function is described by a single power law omega(theta) = (0.159 +/- 0.012)theta(( 0.75 +/- 0.05)), corresponding to a clustering length of r(0) = 3.3 +/- 0.8Mpc h(-1). We find that the most luminous Ha emitters at z similar to 0.2 are more strongly clustered than the relatively fainter ones. The L-H alpha* H alpha emitters at z similar to 0.2 in our sample reside in similar to 1012.5-13.5M(circle dot) dark matter haloes. This implies that the most star-forming galaxies always reside in relatively massive haloes or group-like environments and that the typical host halo mass of star-forming galaxies is independent of redshift if scaled by L-H alpha/L-H alpha* (z), as proposed by Sobral et al.