What you will get out of '100W' solar panel is very dependent on what is actually hitting the panel surface. So, your latitude and time of year has a LOT to do with it. Obviously how 'clean' the air is will impact what hits the panel surface too. The more atmosphere (higher latitude (in northern hemisphere) and further from summer solstice and further from solar mid-day and you'll have less energy hitting the panel. Altitude also will affect output, top of a mountain will provide higher output than sealevel (at same latitude, time of day, weather etc).
So, lots of variables and one person in one area of the planet will observe quite a difference in their measured output versus another.
A 100W panel should produce 100W under 1 solar 'unit' (nominal 1000W per square metre at sea level), what it produces in real life will depend on the above stated variables.
cheers,
george.