With Arif Babul, I am developing a more sophisticated model of black hole growth and jet production which is embedded within a hierarchically growing population of galaxies. As part of this work we have developed a relatively simple model to compute the rate of spin up (or down) of an accreting black hole and the power of the jets it produces while accreting. The figure shows the efficiency (measured relative to the accretion luminosity) of jets driven out of supermassive black hole plus accretion flow systems as a function of the spin (j) of the black hole. The jet efficiency is divided into contributions from winds driven from the accretion disk, and jets launched from close to the black hole horizon. Points show results from relativistic magnetohydrodynamics simulations, while the lines indicate the results from my model of jet efficiency. This model is simple to implement in analytic calculations but agrees extremely well with relativistic magneto-hydrodynamic calculations.