Solar Panel Testing How-To Guide
This instructional post not only shows how to test any solar panel but also what the tests mean in practical terms. The tutorial covers the basics of multimeter use up to the importance and practical use of voltage-current curves and the concept of MPPT (Maximum Point Power Tracking).
Actual tests will be carried out on 100 watt flexible monocrystalline solar panel and a Dokio 80 watt folding solar panel using a 12 volt water heater as a load to determine voltage and current characteristics.
I’m also going to carry out a couple of other interesting tests that you won’t find anywhere else using a buck converter.
- Flexible solar panel (unbranded) advertised as 150 watts.
- Folding flexible portable solar panel – 80 watts (Dokio brand)
- Fluke multimeter
- D.C wattmeter (also reads volts and amps)
- 0-60 volts 20A buck converter
- 140 watt d.c. load
- Solar power meter (reads solar irradiance)
How to measure power output of solar panel
Although this is easily done with the right test equipment, most people don’t simply buy test equipment if they aren’t a professional panel installer. Homeowners just need to know if a panel is working, how much it is generating and is the power rating according to the supplier’s or manufacturers specification.
I bought this unbranded flexible solar panel from a Chinese supplier – I’m not knocking Chinese suppliers at all, I’ve dealt with very good ones, by the way. The first thing I did when the panel arrived was check the panel dimensions. I should have done this at the order stage, but I was seduced by the low price!
How To Check Solar Panel Quality – Real world solar panel output
My flexible solar panel dimensions were given as 1160mm long x 540mm wide for a stated power rating of 150 watts. I assume this measurement is from edge to edge and not the active cell area, but let’s take those dimensions as correct. Panels are rated to a standard called STC (Standard Test Conditions) which is basically in laboratory conditions that are quite rare in many parts of the world.
STC Solar Rating
Among other things STC assumes an irradiance value of 1000 watts per square meter (w/m2). The efficiency for this solar panel was given as 20% by the supplier, so a 1 meter square panel should generate 200 watts.
If we take the STC irradiance and multiply it by the area of my panels at an efficiency of 20% then the result would be the panel STC rating in watts:
- 1.16 m x .54 m x 1000 watts x 20% efficiency = 626.4 x 1000 x 20% = 125 watts
So even at the unrealistic STC rating, it’s easy to see this solar panel isn’t anywhere near 150 watts. There is another rating used that reflects real-world conditions and this is called NOCT (Nominal Operating Cell Temperature). This test standard specifies a panel output rating measured at an irradiance of 800 watts/m2:
- 1.16 x .54 x 800 x 20% eff = 100 watts
NOTE: I’m ignoring other test condition parameters such as temperature rise and atmospheric conditions. The table below gives you some idea of the difference in power output when using the different testing standards. Under STC used my most manufacturing in their advertising the rating in watts is up to 25% over-rated.
Life in Years
STC Rating Watts
NOCT Ratng Watts
SW 255 mono
Panda 60 Cell
It pays to look at the actual spec sheet panel and checkout the NOCT rating. The table below shows that the NOCT power rating at 184 watts is 28% less than it’s STC rating of 255 watts!
So 100 watts seems to be a realistic assessment of what this flexible solar panel may put out – the live tests will show us more. I’ll start of with the basic test for voltage and current.
Digital Multimeter Basics
It isn’t necessary to cover all the functions of this very versatile tool as we don’t need most of them for these tests. The instrument I use is an old Fluke meter which I’ve had for 15 years now – the brand stands the test of time! I’ve dropped it and used it in some quite harsh industrial environments. I can recommend the brand (no, I don’t get anything from them).
Don’t worry – some multimeters are more complex than others but the basic functions are easily found.
How to measure dc current with a multimeter
Discover your solar saving potential
Most multimeters (if not all) have separate test lead entries for volts and current. There’s a reason for this and also an inherent danger. When leads are connected in the Common and V terminal entries, the internal resistance is very high to ensure that a tiny current flows, just enough to engage the electronics that display voltage.
That’s one reason why you don’t see sparks if you connect it to a power supply – there’s very little current flows. The leads should be connected in series with the load. My meter has a fused setting of 10 amp maximum, which is perfect for my solar panel tests.
How to measure dc voltage with a multimeter
From the image above you can that voltage is measured by connecting across the load and current measured by connecting the load in series with the instrument.
It’s important to remember to change over the lead when you change from current to volts. The internal resistance of the current setting is very low. If the leads are connected across a supply a huge current will flow – it will spark and the fuse will blow.
Solar Panel Voltage And Current Output Voc & Isc
Voc stands for the Open Circuit Voltage measured when the panel isn’t connected to anything. Isc is the Short Circuit Current that flows when the panel leads are shorted together. It never sounds right to short circuit any wires coming from a power supply but in the case of a solar panel no long-term harm is done. Strangely enough, when the leads are short-circuited the voltage from the panel is zero – let’s take a closer look.
The figure below is Solar Cell I-V Characteristic Curves showing the current and voltage ( I-V ) characteristics of a particular photovoltaic ( PV ) cell, module or array giving a detailed description of its solar energy conversion ability and efficiency. Lets discuss in some detail.
The image shows the relationship between current and voltage of a typical silicon solar cell cell with normal operating conditions. Solar cell watts or power is current multiplied by the voltage or I x V. If multiplying V and I at various levels from short-circuit to open-circuit conditions, it’s possible to illustrate graphically what the power generated is.
There won’t be any current when the voltage is maximum because these are open-circuit conditions. There’s no circuit connections allowing current to flow. When a current does flow, it starts to drag down the solar panel voltage.
If we now connect the positive and negative leads together the current will be maximum – this is Isc – and the voltage will be zero. The situation is completely reversed.
- Max volts x zero amps = zero watts
- Zero volts x max amps = zero watts
Logically, there must be values of voltage and current when maximum power is generated called Vmp and Imp. This is known as the maximum power point (MPP) of the solar panel and is it’s best power rating for any irradiance.
There are fluctuations according to temperature but basically Vmp is between 0.8 to 0.9 of the open circuit voltage and Imp between 0.85 and 0.95 of the short-circuit current.
Before you start testing your panel
Check the voltage (V) and current (A) ratings of your panel to see what readings you can expect. You’ll find these on the label on the back of the panel usually.
It might seem obvious to say but you need sunny conditions to get the maximum power possible in your location. There’s little to be done to increase the irradiation value, unless you use a mirror or two to increase the amount of sunshine energy falling onto the panel surface to increase it’s efficiency.
Do mirrors increase solar panel output? This will be the subject of another post in the future – search the site to find it.
If your panels are connected to a working installation disconnect panels from system completely. First disconnect the panels from the solar charge controller, then disconnect the controller from the battery.
A word of caution – when exposed to light, solar panels produce electricity so it is recommended that you cover the front of solar panel. If you have a string of panels connected together in series the resulting voltage could be quite high and give a substantial shock or burn. Be careful not to short-circuit the battery bank, if fitted.
Standard Test Conditions (STC)
STC is the most common set of criteria used by manufacturers to market panels to the public. Voltage and current change based on temperature and the intensity of light falling on the surface of the panel. It’s important to have a set of standard test conditions so that different panels can be compared.
The STC standard specifies a temperature of 25 degrees C, light intensity of 1000 watts per square meter and the sun’s angle at 90 degrees to the solar panel at 500 feet above sea level.
Normal Operating Cell Temperature (NOCT)
NOCT is more representative of what you can expect to see, and gives you power ratings that reflect actual conditions. Instead of 1000 watts per square meter, it uses 800 watts per square meter, which is closer to a mostly sunny day with dispersed clouds. The temperature standard is also reduced to 20 degrees C.
Solar panel open circuit voltage test
Voc tells us if the module is working at all. It isn’t a check on functionality or efficiency as such. It simply confirms that all the solar cells are indeed connected together. It’s worth noting that a poor connection my fail, or have a high resistance under load, and this would reduce the panel’s power output.
Open circuit voltage is how many volts the panel has with no load on it. If you just measure with a voltmeter across the plus and minus leads, you will read Voc. As the solar panel isn’t connected to anything, it can’t produce current.
This is the maximum voltage that the solar panel generates according to STC conditions. It’s useful for knowing the maximum number of solar panels to connect in series – inverters and charge controllers have a maximum input voltage that shouldn’t be exceeded.
Why is 12v solar panel Voc higher than 12 volts?
The Voc is always higher for any power source. A typical single crystalline cell generates about 0.5-0.6 volts, so around 36 cells give about 20v open circuit voltage (Voc). The table below shows Voc for panels with different numbers of solar cells:
Number of solar cells
Voc (Open Circuit Volts)
What happens to panel voltage as it delivers amps?
Once the load is connected the current starts flowing and the internal resistance of the power source causes a drop proportional to the current. In the case of solar panel, the resistance is dependent on how exposed it is to the sun. The 20v open circuit voltage drops to 12–14v depending on the load and if there’s enough sunshine.
Depending upon the requirement panels are connected in series, parallel or sometime in a series-parallel configuration.
Connecting Solar Panels In Parallel
Parallel connections are usually used in smaller basic systems, and mostly with PWM (Pulse Width Modulation) solar charge controllers. Connecting solar panels in parallel increases the current the amps and the voltage across each panel stays the same. I use this configuration with my solar kayak setups as it allows me to use basic 12 volt batteries and components, such as 0 to 12 volt motor speed controllers.
The disadvantage of this connection is that high amperage is difficult to transmit over long distances without using very thick wires. At 12 volts, high currents could cause significant voltage drop and lose power in the transmission (as well as possibly overheating cables.)
A system of 1000 Watts could generate over 50 amps which is a substantial current to transfer, particularly if the control panels are several feet from the controller. Thicker cables would be needed so there is a cost impact.
Parallel Solar Panel Connections
Join the positives of the panels together, and the negatives of each panel together as shown above.
Solar Panel Series Connections
By joining the positive of one panel to the negative of the other panel together a series connection is established. Series connections are mostly used in smaller systems with an MPPT (Maximum Power Point Track) Controller. A major disadvantage of series solar panel connection is shading. If one panel’s output is reduced due to its surface being covered, the output of the whole string will be affected.
Solar panel short circuit current test
This test is performed to check the degradation of the solar module – it’s a direct test of the solar panel’s ability to deliver power. You need to check that the maximum current possible (taken from the panel label) doesn’t exceed the multimeter’s maximum current reading. Connection should be quick to minimize any arcing – very possible with high-power panels.
The Isc Test should be done on the single panel on a sunny day and the current kept around 10 aps or less.
The resulting current reading is directly proportional to the amount of sunshine and it’s a great idea to use solar radiation or power meter to measure the sun’s energy. If the value is particularly low, it can be used to adjust the readings obtained when determining the power rating.
Using the 10 amp setting on your multimeter connect each lead to the positive and negative terminals of the panel. Expect to see a value of not less than -20% of the stated rating after adjusting for insolation conditions.
Can a solar panel be short-circuited with no damage?
A solar panel won’t be damaged by a short circuit. Solar panels are designed to be continuously operated at very close to their short circuit current. This is a good, easy test of a solar panel. The optimum operating point of a solar panel is typically about 90%+ of its short circuit current and about 70% to 85% of its Voc (open circuit voltage). This is when it delivers the most power.’
100 watt Flexible Solar Panel Tests
This is a flexible solar panel bought from a non-branded source on Aliexpress. it was very cheap and advertised at 150 watts with and efficiency of 20%. I doubted this immediately due to the dimensions of 1160 mm x 540mm which brings the area in at approximately 0.63m2.
The Voc measurement is never very interesting and not much practical use IMO, but it test at 21.85 volts.
Assuming a lab level of irradiance for STC standards of 1000W/m2:
- 1000 watts x 0.623 m2 x 20% = 125 watts
So even assuming laboratory conditions this panels is never going to deliver 150 watts. Under the NOCT rating system using irradiance values of 800W/m2:
- 800 watts x 0.623 x 20% = 100 watts
This is more like it but this also assumes that the manufacturer’s 20 % efficiency is correct too. How do you really know what this panel can output in watts? The only way is to measure and know the actual irradiance with a meter.
This value for any location is quite difficult to find and most installers use data derived from national solar databases which give them an insolation value for the year or day, and allows them to work out their solar array yield in watts over the long term. This doesn’t really help us to assess an individual panel’s stand-alone performance in real-time.
While it’s true that Isc is an indicator of the current at maximum power I’m looking for a reliable direct method, so I tested the irradiance level in my location as a starting point:
- Instant irradiance mid-day (sunny) Algarve, Portugal, February – 365 watts/m2
I used this as a quick initial calculation. The panel spec sheet indicates that Imp (maximum power current) is 91% of Isc.
- Measured Isc = 3.15 amps
- Imp = 3.15 x 0.91 = 2.87 amps
The spec sheet also states that the voltage at maximum power = 16V, so the power rating of this panel at an irradiance of 335W/m2 is calculated to be:
- 2.87 amps x 16 volts = 34.44 watts
If we adjust this for STC and NOCT standards to find max possible power output we get:
- For STC – 34.44 watts x 1000/335W per sq meter = 102.8 watts power rating
- For SOCT – 34.44 watts x 800/335W per sq meter = 82.25 watts power rating
This means that for general conditions in a realistic setting this panel will only produce 82.25 watts. The maximum power possible form this panel at NOCT irradiance of 800W/m2 is 93 watts. However, there’s still a bit of conjecture and assumption here. How can we test this panel for real?
Solar Panel Check Up – Real-time Solar Panel Power Output Test
The image below shows the test setup. The panel is connected through a buck converter to a d.c. load, which is a water-heater rated at 12V 140 watts – it will try to pull a current of 11.67 amps (watts/amps).
Flexible solar panels result:
- Voc = 21 volts
- Vload = 6.5 volts
- Iload (12 volts side) = 4.95 amps
Power delivered = 4.95 x 6.5 = 32.2 watts – with irradiance of 335 watts/m2 this makes complete sense as the panel is rated at 100 watts at 1000w/m2.
The difficulty is that a solar panel isn’t a constant voltage supply, but a variable one that changes with irradiance and also current, which pulls V down. The above method gives us a pretty good assessment of actual conditions.
Are Folding Solar Panels Worth It?
The Dokio range of monocrystalline folding solar panels are well-known for their efficiency of around 25%. The 80 watt model shown here has an active area of 570mm x 540, so less than a third of a square meter is pushing out 80 watts, plus or minus 3%, with an STC rated irradiance of 1000W/m2.
However, we know this figure is unreasonable – that why we’re testing! Here are the numbers:
The panel spec sheet indicates that Imp (maximum power current) for the Dokio 80W is 95% of Isc.
So, for a measured irradiance of 335W/m2:
- Measured Isc = 1.57 amps
- Imp = 1.57 x 0.95 = 1.5 amps
Vmp (maximum power voltage) is given as 18v, max power for this panel at 335W/m2:
- 18 volts x 1.5 = 27 watts
Adjusting for STC and NOCT:
- For STC – 27 watts x 1000/335W per sq meter = 80.6 watts power rating
- For SOCT – 27 watts x 800/335W per sq meter = 64.5 watts power rating
When are solar panels most efficient?
All solar panels are most efficient when at an angle perpendicular to the sun’s rays at mid-day in the hottest season. This ensures the maximum amount of irradiation in watts/m2 which is directly proportional to the amount of electricity generated in watts/m2.
Why are solar panels so expensive?
Solar panel manufacturing processes are all inherently complex and involve expensive materials. Monocrystalline panels are made from a single ingot of silicon crystal which is slowly grown in rod form and sliced to produce the solar cells for the panels.
Why can’t solar energy be stored?
Solar energy can only be stored in a transformed form such as heat or electricity. There is no known method of storing actual sunlight, or the radiation from the sun.