STC, NOCT, and NMOT: Sean White Decodes Test Conditions Used on the PV Module Datasheet

Every PV module datasheet lists numbers like open-circuit voltage (VOC), max power voltage (VMP), and short-circuit current (ISC), but a module rarely actually operates at any of them. Differences in temperature and sunlight cause voltage and current to skew one direction or another. That’s because the numbers on the datasheet are measured under a specific set of lab conditions: Standard Test Conditions (STC), which set the cell at 25°C, full sun at 1,000 watts per square meter, and a defined light spectrum (AM 1.5).

We sat down with Sean White, HeatSpring’s NABCEP Prep instructor, to walk through what those datasheet numbers actually represent; including why open-circuit voltage needs to be corrected upward for cold temperatures, why PV modules are current-limited sources, and how alternate ratings like NOCT and NMOT give a more realistic picture of real-world performance.

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Transcript

Let’s talk about a datasheet. You’ve got a PV module, you look on the back, and here’s what’s kind of weird: the way your PV module operates most of the time is well below all the numbers on the back of that module. And in extreme conditions, it can actually run higher than those numbers. So what’s that all about?

Take open-circuit voltage, VOC. That number is measured at Standard Test Conditions (STC): which is 25 degrees Celsius (77 Fahrenheit), 1,000 watts per square meter, and a specific light spectrum called AM 1.5 that filters the sunlight. If your VOC is a certain number on the label, you actually have to correct it upward for the coldest temperature you’ll see, because it’s always going to get colder than 25°C, even in the hottest place on the planet. So you always do a temperature correction that makes your real-world VOC a bit higher than what’s printed on the label.

But most of the time, the module is operating below VMP (voltage at maximum power). Now let’s talk about current. There’s maximum power current, also measured at standard test conditions, and usually it’s not as bright out as a full 1,000 watts per square meter of sun. It can get that bright, but conditions are usually somewhat less intense.

Then there’s short-circuit current, ISC. We size our wires based on correction factors applied to short-circuit current, and that’s a little unusual for PV, because PV is a current-limited source. That’s something electricians coming from other trades aren’t always used to – working with current-limited sources. Inverters are current-limited too: if you try to pull more current than an inverter can supply, it just shuts off, because it can’t give you more than it has. A PV module works the same way — it can’t give you more current than the available sunlight allows. To get more current out of it, you’d have to do something like shine a bunch of mirrors on it, or move it to Mercury, where you’d get a bit more current simply from being closer to the sun.

There are some other test conditions worth knowing, and they’re more realistic than STC. One is NOCT (nominal operating cell temperature). It’s similar to STC, but instead of a cell temperature of 25°C, it uses an ambient temperature of 20°C, and the cell heats up well above that ambient temperature – sometimes 25 to 30°C hotter – because something bright is hitting the solar cell.

Here’s something I think is really cool about that: thermodynamics, which basically comes down to the conservation of energy. You’re only ever converting energy from one form to another – module won’t heat up as much when the sun hits it. NOCT is calculated with the module turned off, so it heats up more than a module that’s actively exporting power, especially a very efficient one. If a module is 20% efficient, there’s 20% less energy available to heat it up, because that energy is going out as electricity to the grid instead.

That’s part of why there’s another rating you’ll see on datasheets a lot these days: NMOT (nominal module operating temperature). NMOT is measured with the module exporting electricity to the grid, so it doesn’t heat up as much as it does under NOCT, which means the power and voltage specifications under NMOT tend to look a little better than under NOCT.

So when you’re out in the field looking at modules, wiring them into a string, and measuring voltage,  say you’ve got 10 modules in series and you’re not sure they’re actually all wired in series. Maybe someone wired it wrong. If each module is rated at 40 volts open-circuit at standard test conditions, you’d expect 10 in series to read 400 volts – 40 volts per module. If you measure 400 volts, you can be reasonably confident there really are 10 in series. You might also need to measure the temperature out there, since that affects the reading too. 

That’s the kind of thing you need to know when you’re doing maintenance on a PV system – really understanding what these labels are telling you.