Reality Check

I was disappointed that my adjustable declination feature may not provide much energy gain. So I devised this simple reality check to see if the tilt angle equation was anywhere near accurate. This procedure was based on power delivered during the period April 18 through April 23, and on April 21 in particular.

This exercise will attempt to show reasonable alignment of predicted performance with observed performance. Some justifiable approximations and assumptions must be made, so it is not a formal proof - just a reality check.


When the system was activated on April 11, the array tilt angle was set to 23.6 degrees. And there it stayed. From the daily angle chart, this would be the optimum setting for day 98 - about April 9.


Based on the daily energy chart we might expect about 0.6377 kWh per sq. ft. incident on the array during day 98. We might expect  0.6350 on day 110, or perhaps slightly less due to departure from the optimum tilt angle that was set for day 98.


Between April 18 and April 23 (days 101 and 112) we had six nearly perfect cool, clear days during which the system delivered an average of 42.1 kWh per day. The best day was April 21 (day 110), cloudless from dusk to dawn, on which 43.5 kWh was delivered. Energy delivery is measured by a private kWh meter.


The active area of a KC-120-1 PV panel is 8.41 sq. ft., and 72 of them present a 605.52 sq. ft. target. From the day 110 prediction of 0.6350 kWh per sq. ft. total incident energy, we should have been exposed to 385 kWh. Applying conversion efficiencies of 13.53% for the PV panels and 88% for the inverters, we might expect slightly less than 45.80 kWh to be delivered.


72 x 8.41sf x 0.635kWh/sf x 0.1353 x 0.88 = 45.80kWh

This is remarkably close to the 43.5 kWh actually recorded on day 110.

From the foregoing, the tilt angle equation seems to be aligned with the PTC power rating and measurements of actual energy produced. This gives credibility to the equation, but does nothing to ease my disappointment with the minimal effectiveness of seasonal elevation adjustment.