Very precise position motor recommendation
Since it involved a very precise angle calculation


Exactly what do you mean by "precise"? Keep in mind that the power from a solar cell is proportional to the cosine of the error angle - that is, maximum power is produced when the panel is perfectly perpendicular to the incoming light.

So how does error affect you? The cosine of 1 degree is 0.99984. A one degree alignment error will reduce panel power by .00016, or about .016%. As a matter of fact, an 8 degree error will only drop your power by 1%. Is this really cause for "a very precise angle calculation" ?

Another thing to consider is that, in principle, you need rather high torques available to deal with balance/wind loads. Since you don't need to do high-speed slewing (worst-case is 15 degrees per hour), the obvious approach is a rather small motor with a cheap, low-resolution encoder coupled to a rather high ratio gear box. Both torque and resolution are multiplied by the gear ratio. For that matter, unless you can justify angle precisions much less than 8 degrees, you can couple a rather coarse encoder to the panel rather than the motor, and not worry about losing track of rotation counts.