All solar panels are measured in watts (watts = amps X rated voltage). The watt rating tells you how much power the solar panel will produce in full sunlight.
To find out what wattage you need your solar panel(s) to create, multiply the watt rating of each device you plan to use in a day by how many hours you plan to use it. Add all of these numbers together. Now you need to have at least a rough idea of how much direct sunlight your panel will receive each day. Keep in mind that you should get a panel that supplies more than your actual needs (this will help you get through cloudy days, etc.) This will only give you an idea -- for a better idea, talk with a specialist.
Keep in mind that placement of your panels (and where you live, etc.) can affect how much energy they produce. Also, the wattage label given a solar panel is based on industry standards, and you can actually expect to get around 10% less energy than the label tells you.
The wattage of a panel generally determines its overall size (of course, whether or not it is thin film, single crystal, or polycrystalline can also determine the size of the panel).
Generally, although certainly not always, panels of lower wattages up to 130 watt solar panels are 12 volt.
You can start out with just one 130 solar watt solar panel and add more panels later, as needed, creating a whole solar array.
