ASK THE EXPERTS: Improving a PV System’s Performance

Ben's array also provides shade.

My PV modules are installed at a 10° tilt here in the Applegate Valley in southern Oregon. I know this is a much lower angle than is optimal. I am wondering if either flat or parabolic mirrors mounted at a downward angle at the top end of the array would improve its performance. If so, do you have any suggestions as to where I would obtain appropriate mirrors?

Walter Lindley • via email

My array is also mounted at a 10° tilt, and I’m near Talent, Oregon. When I ran my specs through NREL’s PVWatts Calculator (, the estimated loss (compared to an “ideal” annual tilt of 32°) was about 6%. This is based on a batteryless grid-tied system that can take full advantage of our long, sunny summer days. (See “Sun & Shade with a Double-Duty PV Pergola” in HP182 for a full description of my system.)

In its first full year of operation, my system has produced more kilowatt-hours than projected by PVWatts. That could be due to many factors, including microvariables and defaults in the PVWatts parameters; additional production from my bifacial modules; or the possibility that we had more sun-hours last year than average.

If you have a battery-based system, tilt matters more, since you’ll likely need good winter production. You could compare the cost and complexity of adjustable-tilt racks with the cost of increasing your PV capacity by expanding your system with more modules. Be aware that past attempts at focusing insolation onto PV tended to drive up the cell temperature, thus decreasing efficiency. Some concentrating PV modules even burned their cells to the point of damage. Focusing requires dual-axis tracking, which keeps the focal point on the cell; even simple reflectors would be pointing the wrong way for most of the day. This scheme is perhaps more trouble than it’s worth. And even simple mirrors may void the warranty on your PV modules. Adding PV capacity to your system would be much more straightforward and eliminate the risk of damaging your modules.

Ben Root • Art director, Home Power magazine

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