Checking the health of solar panels with an analysis of the 'I-V curve'

Warning in advance: this article is mainly interesting for solar panel geeks! Because an I-V curve diagnosis is far too complicated for the average owner of a solar system. But since the latest SMA inverters offer the possibility to perform this type of diagnosis, we'd like to explain what it exactly means: for the solar enthusiast.

The I-V curve depicts the relationship between I (current, in Ampere (A)) and V (voltage, in Volt (V)) for a solar panel or a series of solar panels connected together (a 'string'). The current (I) is displayed on the Y-axis, and the voltage (V) on the X-axis. The line generally looks something like this:

Normal shape of an I-V curve

The line intersects the Y-axis at the point where the voltage is 0 Volts. This is the case when there is no voltage difference between the + and — sides of the string, for example when the two ends of the string are connected to each other; at that moment the string is short-circuited, and the maximum possible current flows through the string: the short-circuit current (or ISC, for Short Circuit). In the graph above, this is 14 Amps.

The intersection with the X-axis is the point where the current is 0 Amps. This is the case, for example, when the two ends of the string are not connected to anything: no current can flow. At that moment, the voltage between the + and — sides is equal to the ‘open-circuit voltage’ or VOC. In the graph above, this is slightly more than 450 Volts.

Between these two extremes, it is possible to measure the associated current for each voltage, under constant conditions (i.e. the solar radiation and the temperature remain the same during these measurements). This is how we can draw the entire graph.

The power (P, expressed in Watts (W)) that the string can generate is equal to the product of current and voltage, according to the formula P = V x I . At both intersections of the graph with the X and Y axes, this power is equal to zero. But in between, we can draw a curve for the power, with a clear peak. This peak is the Maximum Power Point (MPP). The function of the MPP tracker in the inverter or optimizer is to continuously search for this MPP: in this way, we always get the maximum possible out of the solar panels.

The red line is power (P), i.e. the product of current (I) and voltage (V). There is a clear peak in the power curve: the MPP

But besides finding the MPP point, the I-V curve can also give us a lot of information about the health of the string. A well-functioning string that is lying in the sun will show the ideal shape as in the pictures above. But an I-V curve that deviates from that ideal image can be an indication that something is wrong.

Here are a few examples:

Shade

I-V curve of a string with partial shade

In this graph we see that the current is high at the beginning, but then drops down from a certain voltage. This is a typical picture you would get if there's shade on 1 or a few panels.

This I-V curve may indicate pollution of some of the panels

In this graph we see that the current already starts to decrease at a relatively low voltage, but that this decrease is not as drastic as in the previous graph. This indicates there could be some pollution on the panels, for example a layer of deposit on the panel edges.

In the case of PID the I-V curve becomes slightly flattened like in this graph

Phenomena such as Potential Induced Degradation (PID) cause premature degradation of panels. As a result, the I-V curve is flattened a bit, as in the image above. The MPP voltage shifts to the left, while the open-clamp voltage remains more or less the same.

An I-V curve analysis can therefore reveal a number of things about the health of a string. That is why it can be useful to perform such analyses. The latest SMA inverters offer this option, but not all. For a number of other inverters, this could also be the case with a future firmware update (which takes place automatically via Sunny Portal).

How do we perform the I-V curve analysis?

You can only perform the I-V curve analysis when you are logged into the inverter. To log in, you must first enter the User Interface of the inverter. There are several methods to do this. One of them is described in our FAQ about changing the WiFi password. Another method is via the SMA Energy App: Open the app, click on ‘Settings’ at the bottom right, and scroll to ‘Quick Access via QR code’. Click on that, and then scan the QR code on the inverter: You will then immediately enter the User Interface.

Some inverters do not have WiFi: in that case, you can call up the User Interace via the IP address that the inverter has received from your internet router.

In the User Interface, you can log into the inverter. The installer password is usually your postcode in capital letters, followed by _zf, so for example: 1234AB_zf . Nowadays, there is often a zero in front; then it becomes: 012234AB_zf. Once logged in, you can click on ‘diagnosis’. Here you will find the option ‘start new measurement’:

Click on 'diagnosis' at the top right and then on 'start new measurement'

When you perform the measurement, you will then see the I-V curve for each MPP tracker, as in this example:

Result of an I-V curve measurement on an inverter with two separate strings

Should you perform an I-V curve analysis?

The I-V curve analysis is a nice new tool, but generally speaking it doesn't make sense to perform it randomly. The best way to check that your solar panels are working is to look at the yield. Every month we write the 'solar panel weather' with the average hours of sunshine of the past month. If your yield more or less matches the expectation, then everything is fine. It actually only happens rarely that something is really wrong with a PV installation which wouldn't be apparent from a red light or an error message. The I-V curve analysis is therefore usually only performed by specialists who are looking for specific problems, for example in large commercial solar parks.

Nevertheless, it may be fun for the enthusiast to 'play' with his or her installation and see what is possible. A few more tips: the I-V curve analysis is actually only useful when it's really sunny, otherwise the result is difficult to interpret. And please note: while the I-V curve analysis is running, your system is not producing any energy! Therefore, it would be a waste to constantly run these analyses.