The PV array is made up of 8 PV panels fastened together and each panel is made up of 36 PV cells. The array is a tracking array which means that it follows the sun throughout the day - when the sun is out - so that it is perpendicular to the rays of the sun, thus maximizing the amount of radiation (solar energy) the panel intercepts. The array generates direct current (DC) power which is converted by electronics (inverters) into 120 volt alternating current (AC) power such as is used commonly in houses and buildings.
One of the solar insolation meters is on the PV array, so it measures the energy density the array is actually receiving. The other one is pointed in the direction that would be best (optimum) for a PV array that did not track the sun, but pointed in the direction that would generate the maximum amount of energy possible for such a fixed array.
The data is archived, beginning 07/03/01. A line(record) of data is generated every 10 seconds, and consists of the date of the record, the time of the record, the system power output at that date and time, the tracking insolometer output and the fixed insolometer output at that date and time. The current array temperature will also be displayed but is now only seen as zero. The 10 second samples are an average of 10 one second samples to reduce noise. The PV array has an active area of cells that is about 6.864 square meters. Since the PV cells are ball park 10% efficient in converting solar energy to electrical energy, the amount of energy per unit time (i.e. the power) we get out of the array is about 10% of what the solar radiation on it. The efficiency of the cells depends on the temperature, so the efficiency is not constant. Furthermore the efficiency of PV cells changes as they age.
Notice that on the plots displayed, the power output is in Watts, a unit of electrical power, while the radiation meters output is in Watts per square meter (i.e. they give how "bright" the sun is on the meters, not the total power they see). To put the radiation meter measurements in the same units as the PV array (Watts) one would multiply the meter readings (Watts per square meter) by the array area. Since the efficiency is about 10%, the power output would be one tenth of what the radiation meter on the array reads.
Since we know at any instant what the array power output, the array active area, and the solar radiation ( Watts per square meter) on the panel is, we can calculate the efficiency at any time.
For example the data record for 09/11/01 at 11:00:00 AM is
09/11/2001,11:00:00,623.584,977.188,725.839,0
therefore the efficiency is
Watts output / ((tracking insolation in Watts per square meter) x (area of array in square meter))
= 623.584/((977.188)x(6.864))
= 0.093 = 9.3% efficient.