Quote Originally Posted by Leewei View Post
That 30 degrees Celcius is fair to use -- in fact, it's probably understating the heat our planet radiates into space by a fair amount. Consider the temperature drop each night following sunset.
Much the same way that the summer winter cycle represents a local change in thermal energy storage from the northern to southern or southern to northern hemispheres the day-night cycle represents a local change of thermal energy that is constantly moving in the opposite direction of the apparent direction of the sun.

There's also the matter of how temperature isn't heat, and if we were concerned about heat we'd be using a power unit. That also makes our lives easier, as earth is very close to an equilibrium between heat out and heat in (it's every so slightly off, hence warming), and you can basically neglect everything but direct sunlight for heat in and call it a day.