# Testing Solar Panels Vs Manufacturers Data

Posts: 6Registered Users ✭✭
Hello,
A few questions for the people out there who are more intelligent than I am!...

If I test Voc and Isc of a solar panel then use P = VI this should equal the power of the panel right? The numbers look about right on the testing I have done when Irradience is around 750 and above.

If this is the case, then why, when you multiply Voc and Isc to find Power in Watts from the data sheet of a panel it is always much higher Power than the rating of the panel?  Do the values of Voc and Isc on datasheets happen on different tests rather than concurrent measurements?

So when I test a panel to see if it meets manufacturers specification is it fair to compare (measured Voc) x (measured Isc) = Power with the Rated Power of the panel?

Craig

Welcome to the forum Craig,

• Power = Vmp * Imp
Voc is Voltage Open Circuit... No current is flowing
• Power = Voc * 0 amps = 0 Watts power
Isc is current short circuit.. Or 0 volts on the solar panel:
• Power = 0 Volts * Isc = 0 Watts power
Note that solar panels are not "solar batteries". Batteries are devices that (attempt) to hold stable voltage from 0 to maximum current the battery is capable of... These are called "voltage sources" in electronics.

Solar panels are (more or less) current sources. The current available from the solar panel is proportional to the amount of sunlight hitting the panel (Imp is "noontime' equivalent sun at ~1,000 Watts per square meter). If you have 1/2 the sun (a bit of haze), then Imp is now 1/2 as much current.

If you want to better understand how solar panels work electrically, this thread has lots of useful information:

http://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 6Registered Users ✭✭
Thank you for such a concise answer and explanation of solar PV vs battery.

With the linear relationship between irradiance  (e.g. 1000W/m2) and current, is it fair to assess your solar panel performance using measurement of sunlight to adjust the Isc value published in your panel data sheet?  For example:

From my panel data sheet I find that:
Isc = 9 amps (at STC, which is 1000 W/m2)

I use a meter to measure irradiance as 750 W/m2 (at the same angle as the panels)
Expected Isc measurement in this light condition is therefore: 750/1000 x 9 = 6.75 amps

I measure my Array Isc Amps in the same 750W/m2 condition and I get 6.5amps.
So my array is performing 0.25 amps below nominal which is about a 4% deficit at this light condition.

Manufacturers tend to have guarantees based on a linear pattern I.e. this panel will have a linear degradation over 20 years to 80% of original performance.

If my solar array is 10 years old then the manufacturer has committed to a maximum of 10% drop in performance.  Since mine has only 4% then I am happy with the performance of my array!  (Or, at least I would be if I hadnt just made these numbers up!!)

Does that sound like a reasonable test?  I am just wondering how you actually can tell if your array is meeting manufacturers performance spec.
• Posts: 6Registered Users ✭✭
I think I've answered my own question today.  I tried the above method with my own panels at home and it has worked out within half of 1% when you use the adjustment of current due to cell temperature.

Next I am onto Voltage.

I have the equation that says
BetaVoc x (Cell Temp - 25) x Voc = change in voltage from Standard test conditions with respect to cell temperature (you can measure this or calculate it using air temp).

When I do this it doesnt seem to have any correction for irradiance.  But when you look at the graphs of current vs voltage then there is a change in Voc measured for both temperature and irradiance. (the dotted lines move to the left on the X axis as the irradiance is reduced)

anyone know how to adjust Voc for irradiance?  it doesnt look like a huge amount

• Posts: 6Registered Users ✭✭
https://ac.els-cdn.com/S1876610217347628/1-s2.0-S1876610217347628-main.pdf?_tid=6c09a877-a3c1-4d1b-b9d8-3b8f24dbe801&acdnat=1530555979_44a55c0761c45c0642ea1d3719d37928

I found a paper that shows you how to... or at least the logarithmic formulas needed to solve the Voc for irradience.  Its quite complicated! I guess this is why we normally just correct for cell temperature, because it is easier and more significant.

Thank you and goodnight!!
• Posts: 1,100Solar Expert ✭✭✭✭
I suggest that Voc is rarely relevant and Vmp is what you want to consider for power calculations.
• Posts: 6Registered Users ✭✭
do i need to have the panels under load to measure Vmp?
Basically you need to "curve trace" voltage and current, and plot Voltage*Current (=Power) to find the Pmp, Vmp, Imp for any particular set of conditions.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 1,100Solar Expert ✭✭✭✭
In other words, yes - you need a variable load.  Or use a loaded MPPT controller and let it find Pmp.
• Posts: 6Registered Users ✭✭
SO yeah basically in the UK we do electrical qualifications to teach you about how to calculate what energy the panels should be making for a given irradiance.  However in practice these formulas are simplified so much that they just dont calculate the right figures.
I have measured open circuit voltages and short circuit currents and the relevant irradiance, then used the formulas to adjust the V and I relative to irradiance so that I can compare with manufacturers data.  Didnt work!

Waited until the sunlight in the day was about right (ie 790W/m² with data from manufacturer for 800W/m²) and simply took a measurement then.
All of the panels are within 3% of brand new testing.  The formulas were telling me that they were under producing.

SO i guess the lesson learned is that it is better to test the panels under the correct light condition than it is to try to correct the values mathematically if you want to compare with manufacturers data.

Thanks for the help everyone.

C
• Posts: 1,072Solar Expert ✭✭✭✭
As mentioned earlier, the Vmp and Imp are the numbers you use to calculate power. Unless you can duplicate standard test conditions of 1000m2 irradiance and 25c temp under load you should expect something closer to NOCT numbers

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.