The output of solar panels is temperature dependent—not on the ambient air temperature, but on the temperature of the panel itself. But let us forestall this discussion and look at the standard reporting method.
Solar panel manufacturers rate their panels in accordance with Standard Testing Conditions (STC). The measurements are taken under controlled conditions in a laboratory environment whereby an equivalent of 1,000 watts per square meter of direct sunlight is created. The PV cell temperature is maintained at 25oC (77oF), and the air mass is 1.5. Now, let me reiterate, the cell temperature is maintained for testing purposes, in a “controlled environment.”
Under more realistic conditions, the temperature of a panel on a day when the ambient air temperature is 20oC (68oF) averages 48oC (118.4oF). So what? Well, for every 1oC (33.8oF) panel temperature rise, there is a reduction of 0.25% efficiency for thin-film cells and 0.5% efficiency for most crystalline cells.[v] So, if a panel is tested at a PV temperature of 25oC (77oF) while the average temperature for a crystalline PV cell at an ambient temperature of 20oC (68oF) is 48oC (118.4oF), there is an efficiency loss of 11.4 percent. For a 200 watts-peak (W-p) STC rated PV panel, the actual output would be 177 W-p. But it doesn’t stop here. Ambient temperatures in summer, warm climates are likely to exceed 35oC (95oF) which is a far cry from the 20oC (68oF) test ambient air temperature. In addition to the 11.4 percent efficiency reduction, an ambient air temperature of 35oC (95oF) will increase the PV panel temperature, reduce the efficiency an additional 7.5 percent—for a total efficiency reduction of 19 percent. The 200 W-p, thus, becomes 162 W-p.
According to Amsolar, a supplier of solar panels for RV purchase, cell temperatures have been “measured as high as 70oC (158oF) on a day when the air temperature was only 80oF” when there has not been any air movement on the back of the installed panels. What does this mean? This translates to 150 watts generated by a 200 watt crystalline PV panel (or 175 watts generated by a 200 watt thin-cell PV panel). [vi]
In other words, cell panel temperature has a tremendous impact on watt output. A 200 watt panel only produces 200 watts under controlled conditions. In the real world, the same 200 watt panel will produce considerably less power than that claimed by the manufacturer even under the best of conditions which is on a cloudless, sunny day, at high noon, and during moderate to low air temperatures.
It is truly astonishing that a recent conversation with someone who claimed to have installed solar panels in Alaska further claimed, “Solar energy works at nighttime.” I didn’t know what to say. Ignorance is bliss?futher claimed, “Solar energy works at nighttime.” I didn’t know what to say. Ignorance is bliss?
[v] What You Need to Know About Solar Power. Reviewed: http://www.solar-facts.com/panels/panel-efficiency.php/