  The cosmic history of supermassive black hole (SMBH) growth is important for
understanding galaxy evolution, reionization and the physics of accretion.
Recent NuSTAR, Swift-BAT and \textit{Chandra} hard X-ray surveys have provided
new constraints on the space density of heavily obscured Active Galactic Nuclei
(AGN). Using the new X-ray luminosity function derived from these data, we here
estimate the accretion efficiency of SMBHs and their contribution to
reionization. We calculate the total ionizing radiation from active galactic
nuclei (AGN) as a function of redshift, based on the X radiation and
distribution of obscuring column density, converted to UV wavelengths. Limiting
the luminosity function to unobscured AGN only, our results agree with current
UV luminosity functions of unobscured AGN. For realistic assumptions about the
escape fraction, the contribution of all AGN to cosmic reionization is $\sim4$
times lower than the galaxy contribution (23% at $z\sim6$). Our results also
offer an observationally constrained prescription that can be used in
simulations or models of galaxy evolution. To estimate the average efficiency
with which supermassive black holes convert mass to light, we compare the total
radiated energy, converted from X-ray light using a bolometric correction, to
the most recent local black hole mass density. The most likely value, $\eta
\sim 0.3-0.34$, approaches the theoretical limit for a maximally rotating Kerr
black hole, $\eta=0.42$, implying that on average growing supermassive black
holes are spinning rapidly.
