The basic desire would be to keep all eight arrays perpendicular to the sun. However, it's a bit more complicated due to other, more detailed requirements.
In the not too distant past NASA held a coding challenge to address just this problem. Here are the requirements from that challenge:
The amount of power generated by each SAW depends on its orientation
with respect to the sun and on any shadows on the solar collectors.
Code to calculate the power generated at a specific orientation will
be provided for the use by contestants in the off-line
tester/visualizer. In addition to maximizing the total power output
there are some constraints on the possible movements:
Each SARJ and BGA is limited to a maximum angular velocity and to a maximum angular
acceleration.
Each SAW must produce at least some minimum average
power over the orbit (which is different for each SAW).
The sequence
of positions must be cyclic, so it can be repeated on the next orbit.
The maximum amount of BGA rotation is not limited, but exceeding a
threshold will result in a score penalty.
- Some structural members of
the SAW mast (called Longerons) have restrictions on how they can be
shadowed.
Longerons are the four long components of the SAW’s mast. If
a longeron is shadowed for a period of time it will cool and shrink.
If some longerons shrink while others do not, this may stress the
longerons and weaken them. This weakening would eventually lead to the
failure of the mast so this must be avoided at all cost.
Acronymology:
SARJ = Solar Array (or Alpha) Rotary Joint (the joint between the central ISS truss and the outboard truss segments from which the solar arrays sprout)
BGA = Beta Gimbal Assembly (the joint between the array and the truss - allows the array to pivot on its long axis)
SAW = Solar Array Wing
The solar arrays may also be moved to specific positions to mitigate loads caused by docking and undocking vehicles - this is the reason for the rather odd positioning seen in the photos taken from visiting vehicles.
Here is a more technical discussion of such constraints from this paper:
Solar Array Constraints
Solar array constraints fall into the
following categories: power, loading (array and SARJ are treated
separately), contamination and longeron shadowing. The power
availability due to any array will be the maximum if it automatically
tracks the sun, but this is not always a safe mode in which to
operate. If power availability drops too low, some ISS subsystems must
be shut off. As described earlier, the attitude of the station needs
to be changed periodically to account for various events by firing a
combination of different thrusters, which in turn imposes structural
loads on the solar arrays as well as the joints, especially the SARJs.
Further, the thruster plumes and water dumps can cause contamination
of the arrays, reducing their power generation.
Additionally,
differential shading of longerons, which are structural elements that
keep the array blankets in tension, put stresses on the arrays, with
the magnitude of the stresses depending on a complex set of
calculations. An array’s longerons can be shadowed by its own blanket
or those of a neighboring array, the amount of shadowing depending on
the solar beta (the elevation of the sun relative to the orbit plane
of the ISS), and the orientations of the adjacent arrays. The same
factors also impact the power generation by the solar arrays; to
improve power, the arrays should not be shadowed, but to keep the
arrays from shadowing each other or the longerons, they are no longer
in an orientation for producing maximum power.
