STC vs. PTC: Why Solar Panel Testing Matters
December 1, 2015
You’re the proud owner of a new 7,800-watt solar energy system. But every time you check your online monitoring, your system is operating below the full 7,800 watts of capacity.
Then you notice the rating plate on your inverter in the basement says 7,600 watts. What the heck?
I can explain the perfectly legitimate reasons for the discrepancy, but first, I have to go off on a tangent and discuss solar panel testing.
Standard Test Conditions (STC)
A solar panel is first tested right in the factory. As the panel comes off the production line, a worker (or robot) places the panel on a “flash table” and hooks up the positive and negative leads to a measuring device. The panel is then “flashed” with fake sunlight. The connected electronics record a number of performance values including the panel’s voltage (volts), current (amps) and power (watts).
These testing conditions are called “Standard Test Conditions” or STC. But what’s standard about them? Well, the light source is calibrated to a defined set of wavelengths and so that precisely 1,000 watts per square meter fall on the front glass of the solar panel. Temperature is the other key test condition – everything is at 77°F (25°C). The solar cells, glass, aluminum frame, and back-sheet are all at 77°F.
If you haven’t noticed already, these test conditions are nothing like the real world. So why does the manufacturer even bother?
As it turns out, there is quite a bit of natural variation – upwards of 5-6% – in the power output from solar cells and panels, even from panels made in the same production run. The manufacturer uses STC testing to sort panels by power and ensure that similar panels are sold and used together.
For example, let’s say that after a flash test, a panel measures out at 257 watts. The manufacturer will “bin” that panel in the “255 to 259.9 watt” bin. A 263.4 watt panel will end up in the “260 to 264.9 watt” bin and so on. The manufacturer will then sell the 255-259.9 watt panels as 255-watt panels, and the 260 to 264.9-watt panels as 260-watt panels. (By the way, even though the panels come off the same production line and cost exactly the same to manufacture, manufacturers charge more for the higher wattage panels.)
Unfortunately, this testing gives only a rough indication of how solar panels will perform in the real world. That’s where the next test comes in.
PVUSA Test Conditions (PTC)
In the mid-1990s, under the direction of the National Renewable Energy Laboratory (NREL), a set of test conditions were developed to measure solar panel performance under “real world” conditions. The conditions were called “Photovoltaics for Utility Scale Applications Test Conditions” or PVUSA Test Conditions; more commonly “PTC.”