Is there a word to describe the direction in which the sun's main rays come from?

In the northern hemisphere, houses are aligned so that windows face the south. In the southern hemisphere, houses are aligned with windows to the north.

So, for example, we have this building advice:

Based on the movements of the sun, passive solar buildings typically have windows (glazing) on the southern facing side* of the building in order to absorb the sun’s heat energy to warm a building during the winter.

but then this has to be followed by this

In the northern hemisphere, in order to face the sun and obtain maximum solar gain, the windows would face the south. In the southern hemisphere, however, it is opposite, with the windows facing the north in order to maximize solar gain.

https://greenpassivesolar.com/passive-solar/building-characteristics/orientation-south-facing-windows/

Is there a word that refers to this direction without the need to specify which hemisphere you are referencing?

the sun is never stationary and moves from east to west. But the direction from which the sun's main rays come at any given time is sunward

adverb

toward or facing the sun.

adjective

facing the sun

A word you could use may be 'insolated', an adjective derived from the verb 'insolate':

insolate, v.
trans. To place in the sun; to expose to the rays of the sun.
....
insolated, adj. exposed to the sun's rays.

["insolate, v.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/96907?redirectedFrom=insolated (accessed March 05, 2016).]

Your example sentence, edited to suit 'insolated':

Based on the movements of the sun, passive solar buildings typically have windows (glazing) on the insolated side of the building in order to absorb the sun’s heat energy to warm a building during the winter.

Using 'insolated', your follow-up starting with "In the northern hemisphere" has been obviated. The first sentence, as shown above, says it all.

If you think the sentence using 'insolated' might result in fussy questions from its intended audience, for example,

Aren't all sides of the house insolated?

you could add a qualifier. For example, "the maximally insolated side" might be best for your audience, or perhaps "the most frequently insolated side" would be better.

The verb 'insolate' and its various derived forms (examples include 'insolated', 'insolation', etc.) are in common use by those involved with the passive solar market.