The amount of sunlight reaching a horizontal surface at Earth’s surface is called insolation, measured in units of watts per square meter (W/m2). Insolation (power), or insolation accumulated over some amount of time (energy), is affected by several factors, including time of day, time of year, latitude, cloud cover, moisture in the air, and air quality.
Photo-voltaic solar panels (PV) are rated at 1000 W/m2. If 1000 W/m2 hits a 200 watt panel, then it produces 200 watts. If the 200 watt panel receives 1000 W/m2 for 4 hours, then it produces 800 watt hours. This would usually be around the 10am to 2pm time frame. Other hours of the day, the panel receives sunlight at lower intensities, so you may pick up another 100 watt hours over the rest of the day.
The closer to the equator you are, the higher the intensity, the further you are from the equator, the lower the intensity. Close to the equator, it’s common to see insolation values higher than 1000 W/m2, and far from the equator, it’s common to see values much less than 1000 W/m2.
A device for determining insolation values for your location, both instantaneous, and with logging, daily averages, is called a Pyranometer. The following website gives a parts list and instructions (and provides a optional kit) for building and using a Pyranometer. This will help you determine how much output your panels will generate at your location.