In a 2002 paper by Elvis et al, they argue that the total luminosities of quasars (which they equate with the supermassive black holes (SMBHs) found at the centres of most galaxies) amounts to 7-15% of the total luminosity of the universe. They assume that these luminosities are due to accretion of infalling matter.
Recently I suggested in this 2007 paper that SMBHs have a possible alternative power source arising from my proposed graviton decay mechanism. On p. 226-227 of the article, I show that the luminosity of the SMBH in our galaxy should be about 10^43 erg/sec. The total luminosity of our galaxy (assuming about 10^11 stars radiating at about 10^33 erg/sec) is about 10^44 erg/sec. My graviton decay mechanism thus gives about 10% of the total galactic luminosity. There is thus good general agreement between what SMBHs (quasars) emit in my model and what they are observed to emit. Accretion is reduced to a lesser role.
The basic equation of my model is
L = - UH
where L is the luminosity, U is the internal gravitational potential energy and H is the Hubble constant.
I had earlier shown that this same heating mechanism can explain the heat emissions of the Earth and larger planets. In my 2007 paper I showed that it can also account for gravitation.