Actually, watts/amps = volts.The thing to understand is Watts x Amps = Volts
watts = amps x volts; amps = watts/volts; volts = watts/amps
The only other thing I would add is that the panel orientation is also important. You want the sun to be hitting the panels at 90 degrees for max power. If you are planning a fixed installation, it's usually best to orient the panels so they take maximum advantage of the winter sun angle. This will help you gain more power when the days are shorter.