We frequently get asked how to create an IV curve for a solar panel. We show you how to do it with a minimal amount of equipment.

Why Generate an IV Curve for a Solar Panel

We build out IV curves for our own panels because we want to see the real world performance of the panel in both good and poor lighting conditions. Having an IV curve allows us to understand the power output as well as the peak panel voltage. This can help us set the right parameters on our charge circuit to deliver more power to our batteries over the course of the year (see How to Select MPP Voltage).

Our panels are composed of 12 to 36 individual cell pieces that are cut from whole cells. The panels will always under perform any specification based on the solar cell supplier specification sheet for the following reasons:
1. When cutting solar cells, you create microshorts along the edges of the cut cell pieces
2. The output of a string of solar cells provides as much current as the worst performing cell piece – each cell piece will have an output that falls along a distribution curve, which pushes the total panel output lower
3. The coating on the panel further lowers the output

Equipment Needed for Creating an IV Curve

There are some very sophisticated systems for measuring IV curves on large commercial panels that tend to cost several thousand dollars. It is a little more time consuming to do by hand, but very doable. You’ll need:

– Solar Intensity Meter, we like this one
– Load, we use 3644A by Array
– A way to power the Load, either a long extension cord or battery with inverter
– Wires to connect solar panel to Load

Setup for Measuring IV Curves

Before going outside, connect a wire with 12-24″ of length leading to to leads. If your panel has solder pads, it is useful to hard wire cables onto the panel to get a secure connection.

wires attache to solar panels for IV Curves

Wires Attached to Small Panels Make Measurements Easier

Next, you will need a way to:
1. Keep the panel and solar intensity meter perpendicular to each other
2. Adjust the angle and direction of your panel and meter to achieve a consistent solar intensity

We use a shop cart with an adjustable lid. The meter mounts on the side of the lid, pointing toward the sun. The panel lies flat on the lid and is secured with magnets.

Setup for Testing IV Curves – Solar Intensity Meter is Perpendicular to the Solar Panel

A second person is helpful especially when the solar intensity is fluctuating. One person can adjust the panel to keep a consistent solar intensity and the other person can enter the data.

 

Recording IV Measurements

We have a spreadsheet template that we use, but it starts with the panel specification.

Pick a sunny, clear day unless you’re deliberately trying to measure low light performance. If the sun is going in and out of clouds, it will take a lot longer to do your test.

Record the temperature as every degree the panel climbs over 25C will reduce power output by roughly 0.5%.

Point the solar intensity meter and try to get to either 1,000 W/m^2 or its peak if you are unable to reach 1,000 W/m^2 (in the summer in New York, we often peak at 950 W/m^2 because of the humidity). Record solar intensity.

Solar Intensity Reading

Solar Intensity Reading of 967 Watts per Meter Squared. This is 3.3% less than “one sun.”

Connect the panel to the load, record the voltage.

IV curve reading for small solar panel

This reading shows a panel Voltage when the load current is set to 20mA

Increase the current in steady increments and record the voltage. Keep an eye on the solar intensity. If it changes, you need to re-record the solar intensity or adjust the position of the panel to get the same solar intensity.

Eventually, the voltage will drop to 0. This is your short circuit current.

For example, we recently tested a panel rated at 6.2 Watt, 9.8 Volts. Here is the data we gathered.

Temperature = 16 Celsius

Solar Intensity = 1005 W/m^2

Amps Volts Watts
0 11.4 (Open Circuit Voltage) 0
0.2 11.06 2.21
0.4 10.59 4.24
0.5 10.24 5.12
0.6 9.54 5.72
0.61 9.39 5.73
0.62 9.27 5.75 (Max Power Point)
0.63 9.08 5.72
0.64 8.72 5.58
0.65 (Short Circuit Current) 0 0

Note that as we get closer to the maximum power point, we increase the current at smaller increments. The curve looks like this:

IV Curve for Small Solar Panel

In the end, the panel deviated from the rated peak voltage by 4.9%, peak current by 3.1% and peak power by 7.9%

Repeat at Low Light Intensity

Conditions are rarely perfect in solar. We suggest repeating the same test at lower light intensities. You can angle the panels away from the sun on a clear day to get to ~500 W/m^2 or put them in a shady area to get ~100 W/m^2. Both the current and voltage will decrease.

Scale and Report the Results

Once you have the data you can plot the voltage vs current to create an IV curve. Assuming that your power measurements were above 500 W/m^2 range, you can scale up linearly to 1,000 W/m^2 (full sun), just be sure to note how you’re scaling the data.

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