STC, PTC, NOCT: What do they mean and how to use them?
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When you start searching for a solar panel to buy, you may come across some acronyms. You might see them under the solar panel specifications sheet and wonder what to make out of them.
STC and PTC are both test conditions used to rate the performance of a photovoltaic module (PV panel), while NOCT is referred to the PV cell temperature and it’s obtained under prefixed environmental conditions. Of course, it’s not necessary to know what they are in order to buy a solar panel. However, if you want to make a better deal, these parameters are very handy. Let’s see how:
What is STC and what is it used for?
STC stands for Standard Test Conditions and set the base conditions, as reported in the table below, under which a solar panel will be tested.
STC is used by solar panel manufacturers to test and rate their panels. The value that interests us is the maximum power (Pmax) or rated power (Pr), which is the nominal power of a solar panel when you look to buy one. It could also be called peak power.
In a specification sheet, it’s always indicated in a section with STC nominated nearby.
So, from the table above, we can see a peak power of 100 W (watt). It means the panel is sold as a 100 W panel. The problem is that STC values are not representative of the real operating conditions of a solar panel and that means the rated power is overestimated.
In real working conditions, the overall module temperature is usually much higher and could reach 40-50 ⁰C (104 -122 ºF), which drops the performance of crystalline silicon (c-Si) modules by about 0.5% for each 1°C rise in temperature.
So engineers need to develop an accurate thermal model for the solar panel if they want to be able to evaluate the effects of the major meteorological parameters on the final temperature of a solar module and that brings us to NOCT.
What is NOCT and what is it used for?
When the solar energy (Irradiance) is absorbed by a solar panel, it’s converted partly into thermal energy and partly into electrical energy. That means the cell and overall module temperature will increase while producing electricity. However, there are also other environmental factors that influence the PV module temperature.
It is not easy to measure the PV module temperature considering all the meteorological variations. So, scientists defined Nominal Operating Cell Temperature (NOCT) to simplify the calculation.
NOCT is defined as the cell temperature of an open circuit PV module, under conditions that you can look in the table below.
Solar manufacturers include this data in the specifications sheet of a PV panel, which they usually report as NOCT or TNOCT.
Another factor that influences PV module temperature in working condition is the mounting configuration of the photovoltaic array. Therefore, another parameter called INOCT (Installed Nominal Operating Cell Temperature) was defined to help us in describing the cell temperature of an installed array and it’s obtained from the NOCT.
The procedures for calculating more accurate values for PV module temperature in operating conditions are complicated. If you want to learn more, look at the references section at the end.
How to use NOCT to choose a solar panel?
Let’s say that you did your research and found different solar panels with the same rated power that you need. So, other than the manufacturer’s reputation, how could you tell which one is better?
Solar panels efficiencies, for modules with the same rated power, do not help that much. A more efficient solar panel will produce more power per m^2. That means if your space for installing solar panels is limited, then a more efficient panel could be a better choice. Because it will have a smaller size compared to the PV module, with the same rated power, but lower efficiency. The downside is that they are also more expensive.
NOCT will help you to choose the PV panel that will perform better and will give you more power once you install it. As I told you before, the overall temperature of the module will seriously affect its peak power. So to estimate a more realistic value, we have to calculate the module power losses under operational conditions.
We use data that you can simply find in specifications sheet provided by solar manufacturers:
- Maximum Power at STC (rated power)
- Nominal Operating Cell Temperature (NOCT)
- Temperature Coefficient of Pmax
The temperature coefficient expresses the effect of the module temperature deviation from 25°C set by STC.
So the power losses will be given by:
Power Losses (%)= T coefficient x (NOCT – 25 ºC)
Keep in mind, the formula here is a good estimation for small projects. But professional solar installers will use more accurate calculations to find the module temperature.
So for our example, consider:
- 240W rated panel
- Temperature Coefficient of Pmax= -0.485%/ ºC
- NOCT= 47.5 ºC
Power losses (%) = 0.485 x (47.5 – 25) = 10.91%
That means we lose at least 10.91% of the rated power when we start actually using the PV panel. In other words, the maximum power that we can expect from this panel is about 213 W.
Smaller NOCT = Less Power Losses
What is PTC and what is it used for?
PTC stands for Photovoltaics for Utility Scale Applications Test Conditions (PVUSA test conditions) or simply Performance test conditions. These conditions were developed to test and compare PV systems as part of the PVUSA project.
PTC is generally considered as a more realistic measure of PV output because the test conditions better reflect “real-world” solar and climatic conditions, compared to the STC rating.
To give you an idea of the PV panel performance under these two different tests, I selected to random tests from The Go Solar California database.
In the state of California, PV system components for buildings (modules, inverters, and system performance meters) must be certified through the California Energy Commission’s (CEC) PV system certification program. And they require rating PV panels using PTC.
Unfortunately, it’s not an obligation for all PV module manufacturers in the world to do this test and report it on their module specifications data. But, If you find it in their specifications, then you can skip the calculation by NOCT, which I showed you previously, and look at maximum power based on PTC.
One last thing, sometimes you might not find PTC peak power value in specifications sheet. Don’t give up yet! Look at the certifications section. If there is a voice like CEC LISTED, then you can find it on the Go Solar California website, under PV modules list.
The truth is that in “real-world”, the solar system losses are higher. PTC and STC do not take into account all possible factors that influence a PV system performance. Factors such as shading, module mismatch, wire losses, inverter and transformer losses, panel degradation over time, and high-temperature losses for arrays mounted close to or integrated within a roofline vary depending on, for example, season, geographic location, mounting etc.
If you want to get an estimation of your PV system performance, I recommend you to use the PVWatts Calculator provided by the National Renewable Energy Laboratory (NREL).
 Smets, Arno & Jäger, Klaus & Isabella, Olindo & Van Swaaij, R.A.C.M.M. & Zeman, Miro. (2016). Solar Energy – The physics and engineering of photovoltaic conversion, technologies and systems.
 U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy
 Go Solar California