  We develop a new spectral model for the broadband spectral energy
distribution (SED) of Active Galactic Nuclei (AGN). This includes an outer
standard disc, an inner warm Comptonising region to produce the soft X-ray
excess and a hot corona. We tie these together energetically by assuming
Novikov-Thorne emissivity, and use this to define a size scale for the hard
X-ray corona as equal to the radius where the remaining accretion energy down
to the black hole can power the observed X-ray emission. We test this on three
AGN with well defined SEDs as well as on larger samples to show that the
average hard X-ray luminosity is always approximately a few percent of the
Eddington luminosity across a large range of Eddington ratio. As a consequence,
the radial size scale required for gravity to power the X-ray corona has to
decrease with increasing Eddington fraction. For the first time we hardwire
this into the spectral models, and set the hard X-ray spectral index self
consistently from the ratio of the hard X-ray luminosity to intercepted seed
photon luminosity from the disc. This matches the observed correlation of
steeper spectral index with increasing Eddington ratio, as well as reproducing
the observed tight UV/X relation of quasars. We also include the reprocessed
emission produced by the hot inner flow illuminating the warm Comptonisation
and standard disc regions and show that this predicts a decreasing amount of
optical variability with increasing Eddington ratio as observed, though
additional processes may also be required to explain the observed optical
variability.
