Here is a link to a website put together by the California Energy Commission to provide evaluations of solar panel performance:
I find it interesting that there does not seem to be much of a difference between monocrystalline and polycrystalline panels. Both seem to produce about 90% of their rate values under standard test conditions of:
- 20C air temperature
- 1 meter/sec wind speed (2.2mph)
- 10 meters (33 ft) above the ground
- Air mass of 1.5
- ASTM G173-03 standard spectrum
- 1000 watts/sq meter solar irradiance
Ok. So, how is that practical? Well, here is a calculator, based on historical data, of how much irradiance is expected in a given location.
For Dallas in September, expected solar irradiance of a South facing panel is 4.96kWh/sq meter/day. So, it appears that a real world 100W panel should be expected to produce 9% of this amount – or 446Wh.
To put this in perspective… 446Wh would allow a person to use 18.6 watts of electricity constantly over a 24 hour period, assuming that the system has a battery to store energy for use at night and cloudy days.
Note that solar irradiance is about half that amount in the winter.
Bottom line: An ideal 100 watt solar electric system in Dallas allows a person to constantly use about 18 watts of power during the month of September; about 9 watts in the winter. Real world results are almost certainly less.