Watt and voltage for DIY iPhone charger
Voltage does matter, it must be =&lt;5V.  You can use a simple voltage regulator to keep yourself from going over.  You can also wire the panels so that they don't go over 5 (if each panel does 1.5V, 3 panels get you 4.5V, which will work but be slower. 

So the 4 panels will give 6 Volt and the 5 Volt Zener diode will cut off the access 1  volt

Wattage is based on Voltage and Amperage.  The iPad can use up to 2.1 Amps.  Solar panels will always make the same voltage, but the more sunlight the more available amps.  You do not have to do anything to limit the amperage as the iPad will not pull more amperage than it needs.  This is the same way a flashlight works, a battery could dump all of it's power at once, like a capacitor, but the lightbulb doesn't need or want all that, so it draws only small amounts at a time.  This is also why a charged capacitor, which is designed to release current very flash like in a camera's flash, can also power an LED for a relatively longer time.