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DIY Solar Panel Performance Test using Ceramic Disc Rheostat

Determine a solar panel maximum output power performance is important. By measuring the expected maximum power output of a solar panel, we can know that whether the solar panel is producing as expected or under performed. By comparing the measured value and the theoretical calculation, we can know the performance ratio of a solar panel. 

Many people measure open circuit voltage (Voc) and short circuit current (Isc) to determine the performance of Solar Panel. However, this method is partially correct as to measure the performance of solar panel which is not operating or producing power. However, the correcy way would be measuring the maximum operating power (Pmp) value.

In order for load tuning to obtain for maximum power of a solar panel, we need a high power ceramic disc rotation Rheostat. It is similar to potentiometer but with higher withstand power capability. Due to the short circuit and high current operation, the Rheostat receiving large power in term of heat. Thus it is best not to operate too long or may damage the Rheostat. To select a suitable Rheostat, the power rating should be at least two times higher than the Solar Panel wattage. The resistance of the Rheostat should be not more than 20 ohm.

100Wp Solar Panels

100Wp 18Vmp Solar Panel is commonly used for Solar DC direct and small Solar Off-Grid PV System. A good solar panel may last at least 20 years. Grab one at our affiliate link here !!!

Rotation disc sliding Rheostat

This is a high power manual operated (handheld) variable resistor or potentiometer. It is  widely used in auto transformer equipment. It also can be automated by adding a rotating motor controlled by Arduino. Kindly get it at our affiliate link here !!!

DC Energy Meter

DC Energy meter display important measurement units such as voltage, current, power and energy. It is very useful to troubleshoot or analyse an electrical or project performance. Grab one at our affiliate link here !!! 

Hardware Wiring

The Solar Panel is connected to the middle and either one end terminal pin of Rheostat. In between you need a DC Energy Meter for output reading. The Voltage and Current will be adjusted or changed when you rotate the Rheostat. There will be a point where the combination of current and voltage, the power output value is at maximum. The DC Power Meter shall be able to measure low DC voltage (lower than 5V) if the Solar Panel has low operation voltage (Vmp) value.

SM206 Handheld Irradiance Meter

SM206 handheld irradiance meter is a low cost with reliable accuracy that could measure light and solar intensity in W/m2 and Btu unit. You can get it at our affiliate link here !!

For measurement readings, we need to measure 3 things: The solar panel cell temperature, the solar intensity in W/m2 and the operational maximum wattage of the solar panel. Just now we have mentioned the DC Energy Meter and Rheostat in order to obtain the operating maximum wattage. In order to measure temperature of the solar panel cell, it is best to get a temperature gun which can be measured without physically touching the panel. Besides, we need a Solar Irradiation to measure sunlight intensity in order to come out with expected value if the solar panel were in healthy condition.

For calculation purpose, we need also the datasheet of the solar panel. You may need the Maximum Operating Wattage (Pmp), and temperature coefficient of the maximum wattage (Pmp – in %/°C). For easy calculation, I have made a excel sheet and you just need to key in all the values here

Temperature Gun

A temperature gun uses infrared sensor to detect surface temperature without physically intact with the measured body. Grab one for yourself at our affiliate link here !!!

Step by Step

  1. Record the maximum power at STC (Pmp) and its temperature coefficient (Pmp %/°C) values of solar panel from its datasheet. 
  2. Connect the wiring as per diagram. Turn on Irradiance Meter and Temperature Sensor.
  3. Let the Solar Panel facing directly to sunlight or flat surface for accurate reading. Let it sit outside for a while until it reaches equilibrium temperature with the environment.
  4. Make sure irradiance meter and solar panel surface are both facing the same direction and tilt angle and make sure no shadings on both sensor and solar panel.
  5. Tune the Rheostat slowly to get the maximum power reading from the DC Energy Meter. Start to take readings only while the value is stable.
  6. Record the irradiance and temperature values together with the maximum power from DC Energy Meter within 5 seconds.
  7. Unplug the power or tune the Rheostat to the zero output wattage to prevent heat build up within the component.
  8. You can record more readings for an average value. Put all the values in the excel sheet for calculation.
  9. Repeats as you wish.

2 Pin Waterproof Connector

It is the waterproof connector which is suitable to be used outdoor. It can connect up to 1..5mm square cable size. Grab a set of connector at our affiliate link here !!!

Rheostat Selection

Rheostat or adjustable resistor is rated by its resistance value and power dissipation. The resistance value and power dissipation of Rheostat ratings are interrelated to each other. As a rule of thumb, smaller current and higher wattage is preferable to measure high power solar panel.

The resistance value is based on the minimum operation current of the panel. For example, a 100W 18Vmp 5.56 Imp solar panel should need only about 3.2 ohm in overall circuit (including internal resistance of the solar cells), thus external resistance from Rheostat may be lower. If we get a 20 ohm Rheostat, you may need the solar panel to be able to generate at least 0.9A to able for performance test which is about 16% of its full power. This ratio is suitable while maintaining a high power dissipation with low resistance.

The power dissipation rating of the Rheostat is I x I x R. The Rheostat is designed based on its highest resistance rating. For example, a 100W, 20 ohm rating Rheostat would able to withstand current up to 2.2A while 500W 10 ohm rating would be able to withstand current up to 7.0A. Of cause, short time usage with overload current is possible if the overload current is operated less than 5 seconds. In reality, Solar Panel may not operate at maximum resistance of the Rheostat, thus 500W 10 ohm Rheostat should be possible for most commercial large solar panel for non-continuous operation. For extreme high current (typically more than 10A panel), better to use 1000W 10 ohm / 500W 5 ohm or compensate with lower radiance level during test. Just remember to turn down to minimum current right after each test.

Ferrule Lugs and Crimper Set

Ferrule lugs are used at terminals for tight and secure connection. Besides, it also prevent stranded cables accidentally touching adjacent power line that may cause short circuit. Grab 1 at our affiliate link here !!!

Terminals Crimping Plier Set

This is a crimping tool with interchangeable jaw for crimping many types of terminals. It is a must have for DIY electronic and electrical for professional installation work. You may choose at out affiliate link here !!!

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Solar Panel Performance Test.xlsx