Ohm's Law equations are easy to understand if you draw a simple chart. Draw a capital letter "T." On top of the "T," write "Volts." On the left underside of the "T," write "Amps." On the right underside of the "T," write "Resistance." Cover up, with your thumb, the value you want to find out. Volts divided by amps equals resistance. Volts divided by resistance equals amps. Amps times resistance equals volts.
Draw another "T." On the top of the "T," write "Watts." On the left underside, write "Amps." On the right underside, write "Volts." Cover up the value you want to find out. Amperage is wattage divided by volts. Volts are wattage divided by amperage, and wattage is amps times volts.
A motor is resistance. Notice that without resistance, amperage cannot be drawn. Without amperage, wattage cannot be measured.
Solar panels are rated by the total wattage produced. Various resistance loads are placed across the two terminals of a panel, and the voltage output is measured, according to the amperage draw. Remembering that wattage is voltage times amperage, no amperage draw is produced without a resistance load, namely your motor. The more the resistance of the load, the more the amperage draw.
A motor's final speed is complicated, and based on several factors. First, you must understand the difference between no-load and full-load speed. No-load means free running, with no work being placed on the shaft. When you place a load on the shaft, such as a compressor, the motor slows down due to it having to work. In the process, its internal impedance (a form of resistance) goes up. This means the amperage draw is greater. To increase the motor speed to compensate for the load, you have to feed in more voltage to the motor, which increases the amperage draw. The solar panel may or may not be able to add voltage, based on its wattage rating.
Amperage is not an output, but rather an electrical flow that is drawn out by resistance. Amperage draw is dependent on how much effort the motor has to exert. The more the effort, the greater the amperage draw. Wattage is the output, and solar panels are rated in watts. To increase a motor's speed, you have to feed in more wattage, which is volts times amps. A milliamp is one thousandth of an amp. A motor drawing milliamps is not that big, so drawing in more milliamps will not increase its speed, but adding in more wattage will.