  We present the power spectrum of solar irradiance during 1985 and 1987
obtained from the ACRIM project from time scales of minutes to months. At low
frequency the spectra are Lorentzian. At higher frequencies they are
proportional to $f^{-{1/2}}$. A linear, stochastic model of the turbulent heat
transfer between the granulation layer (modeled as a homogeneous thin layer
with a radiative boundary condition) and the rest of the convection zone
(modeled as a homogeneous thick layer with thermal and diffusion constants
appropriate the lower convection zone) predicts the observed spectrum.
