Panel Power Testing

Have you first checked out how Solar Panels Work?

Have you ever asked the questions:

"How much power is my solar panel capable of producing?"

"How many watts is my solar panel producing now?"

It is somewhat difficult, and certainly tedious, to accurately determine the power output of a solar panel without the use of specialised equipment. Luckily, there is another way that you can measure - and then estimate - the power output of your solar panel to within less than 10% of the actual power.

The simple method detailed in this document will yield an estimated maximum power value. Basically, the process is to connect a variable resistor, the load, across the solar panel output, expose the solar panel to Sunlight and measure both the voltage drop across the load (in volts) and the current flow (amps) through the load. You will need to take multiple readings for both these measures as the load resistance is varied from very large to near zero. The measured power, in watts, is the multiplication of voltage across and current flow through the resistive load. Consequently a value of panel power can be calculated.

Note that at infinite resistance, the power will be zero as there is no current flow (open circuit voltage). Similarly, power will also be zero when the resistance is zero (short circuit current) as there will be no voltage drop across the load. As we can never reach either condition we use very large and very small resistances instead to simulate these two end point conditions.

Maximum power will be found somewhere between these end points. With enough measurements taken at various resistance load levels, the maximum power produced can be estimated. Graphing the results gives a clear indication of the maximum power point. (Hint: the maximum power point normally occurs at a voltage somewhere near 80% of the panels open circuit voltage.)

The above method will give an accurate power value at the particular Sun level at which the testing is performed. However, if you wish to know the maximum power the panel is likely to produce at a specified Sun level, say at AM 1.5 which is the specified test level for the Model Solar Vehicle Challenge you need to know the Sun level at which test was performed in relation to the AM 1.5 standard.

In brief, AM 1.5 defines the Sun intensity after the light has travelled through 1.5 times the thickness of the Earth’s atmosphere (this is equal to a 48.2 degrees Sun angle) and is defined as 1000 watts per square metre. This means that it is possible to read more than 100% Sun on your calibrated solar panel depending where you are on the Earth’s surface and what time of year it is. Another reason for a higher reading can be the Sun is shining through a hole in the clouds and there is a lensing effect.

This is where the Scorpio No 10 Calibrated cell comes in. This solar cell is calibrated to read 1 ma short circuit current per 1% of Sun at AM 1.5. Consequently if you take a reading of power on a solar panel and simultaneously take a reading of Sun % using a Scorpio No. 10 calibrated cell you now know the power your panel produces at a particular % of AM 1.5. With this data it is simple to adjust your panels measured power up or down to the power expected at 100% Sun at AM 1.5 conditions using a simple multiplication.

It is imperative that the panel being tested and the calibrated cell are both unshaded and in the same plane when the readings are taken otherwise the accuracy of the results will be significantly degraded.

Extensive testing has proved that, provided the Sun level at which your test is taken is above 50%, the power calculated at 100% Sun by using the above ratio method gives acceptable accuracy, generally within a few percent of the results using VMSVC equipment. The closer to 100% sun the test is conducted, the better will be the accuracy.

But we are still left with the chore of taking readings of voltage and current through the load as its resistance value is varied through the range of very large to near zero.

Or are we ?

Provided you have a good quality undamaged panel, a reasonably accurate estimate of panel power can be obtained by measuring its open circuit voltage (in volts) and short circuit current (in amps) then multiplying these values together and then multiplying by 0.75.

(Open circuit volts) x (Short circuit current) x 0.75 = Power in watts

Again, provided your panel is good quality and undamaged reasonable accuracy will be maintained when a power value obtained by the above method is used with the ratio method to calculate the expected power at 100% Sun.

Caution: A low quality or damaged panel will reduce accuracy as will high panel temperature during testing.

Scorpio Technology No 10 Calibrated solar panel details: This small solar panel allows easy determination of Sun intensity.

It is a valuable tool when testing solar powered vehicles enabling us to take into account the prevailing Sun level during tests. Even better, this panel is exceptionally simple to use to measure your sun level to 1% accuracy.

The panel is calibrated to supply 100 milliamps in full Sun at AM 1.5 conditions, consequently the reading in milliamps is equal to Sun % based on the AM 1.5 datum. Please note AM 1.5 was chosen as it is the standard used for testing commercial solar panels.

For details and ordering the No 10 calibrated cell go to

Set up and use: This is quite simple, it requires a digital multimeter with a milliamp range equal to or greater than 100 milliamps. With the meter set to this range in current mode connect the calibrated solar panel to the meter probes, panel positive lead (which is red) to meter positive probe and panel negative lead (which is black) to meter negative probe. This is all the set up required. When exposed to sunlight, the multimeter will read the solar panel short circuit current which varies directly with Sun intensity, and because the solar panel is calibrated to produce 100 milliamps in full Sun the reading in milliamps translates directly to Sun %, even above 100% Sun levels.