Watts in Solar Panels
Your understanding of Watts is correct.  Power = Voltage * Current. Power is measured in watts, so your "xy" analogy is accurate.

One thing that you should keep in mind is losses that occur between energy conversions. Even if a device is rated at Z watts, it may sometimes draw more than Z (or sometimes less than Z). Try to have a little extra margin on your solar array output wattage and you will be fine. Happy Travels!