Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2), according to Wilkipedia.
While traveling through the atmosphere 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/m². Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum.
In North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day). This represents the available power, and not the delivered power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.
In the Tucson area, solar irradiance is near the higher end of the range. For example, on July 2, 2007, solar irradiance powering the Pennington Street Garage solar system was 859 W/M2 at 1:30 p.m. Daily generation is about 400 kW.