The drawing below shows the basic construction of an axial-flux, permanent-magnet, synchronous generator.
In order to calculate how much power can be generated by permanent-magnet generator, it is necessary to have the following:
Detailed magnetic strength specifications and dimensions of the magnets.
The generator construction and geometry details.
The rotor iron dimensions and magnetic characteristics.
From the construction details, determine the length of the path of the magnetic field. From the path of the magnetic field and the magnetic characteristics of the materials, determine the strength of the magnetic field in the air gap.
For any instant in time, the voltage can be calculated in each segment of wire that the magnetic field passes through from E = v x B x L, where: E is the electromotive force (voltage); v is the velocity of the field; B is the strength of the magnetic field; and L is the length of the segment of wire. For any segment of wire that is not at a right angle to the direction of field motion, multiply by the sine of the angle of the wire with respect to the direction of motion.
To get the total voltage a coil at any instant in time, add all of the voltages of the wire segments. Then determine the waveform from the calculations for an appropriate number of instants in time.
To determine the output voltage of the generator, add the coil voltages considering the scheme selected for connecting the coils together.
There will be some voltage drop and power loss due to the wire resistance. Since the generator will have an AC output, there will be some voltage drop in the inductance of the coils.
The amount current that can safely be drawn will be determined by the losses, the rate at which heat is rejected to the surrounding air and the temperature rating of the winding insulation.
The maximum amount of power that can be generated will be determine by the voltage and current and the available input mechanical power from the system driving the generator.