  Reflected light curves observed for exoplanets indicate bright clouds at some
of them. We estimate how the light curve and total stellar heating of a planet
depend on forward and backward scattering in the clouds based on Pioneer and
Cassini spacecraft images of Jupiter and Saturn. We fit analytical functions to
the local reflected brightnesses of Jupiter and Saturn depending on the
planet's phase. These observations cover broad bands at 0.59-0.72 and 0.39-0.5
{\mu}m, and narrow bands at 0.938 (atmospheric window), 0.889 (CH4 absorption
band), and 0.24-0.28 {\mu}m. We simulate the images of the planets with a
ray-tracing model, and disk-integrate them to produce the full-orbit light
curves. For Jupiter, we also fit the modeled light curves to the observed
full-disk brightness. We derive spherical albedos for Jupiter, Saturn, and for
planets with Lambertian and Rayleigh-scattering atmospheres. Jupiter-like
atmospheres can produce light curves that are a factor of two fainter at
half-phase than the Lambertian planet, given the same geometric albedo at
transit. The spherical albedo is typically lower than for a Lambertian planet
by up to a factor of 1.5. The Lambertian assumption will underestimate the
absorption of the stellar light and the equilibrium temperature of the
planetary atmosphere. We also compare our light curves with the light curves of
solid bodies: the moons Enceladus and Callisto. Their strong backscattering
peak within a few degrees of opposition (secondary eclipse) can lead to an even
stronger underestimate of the stellar heating.
