# Solar Panel Voltage test help

Discussion in 'Power Electronics' started by RodneyB, Jan 14, 2017.

1. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
I am trying to follow the voltage on a 20 Watt Solar Panel,

I have it connected to an Analogue to digital converter that reports the voltage as it goes up and down.

I am based in Harare, Zimbabwe, so my solar panel is fixed North facing at an angle of 25 degrees.

As it gets light quite early the panel starts recording the voltage at 05:33 this morning the recorded voltage was 10 Volts.

This however is without a load on the panel so the results I am looking for are not accurate.

I am trying to find the actual voltage being produced with a load so that I can work out the actual peak sun hours.

I am not 100% sure how to do this. Your assistance will be greatly appreciated

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
assuming from missing information that the solar panel is for 12 volt applications - 20 W will be more than 1 amp and less than 2 amps at the rated panel voltage.

a resistance in the 12 ohm range would be appropriate. rated at MORE than 20 watts of course.

put an ohm meter across the heating element of a toaster and see if it is in the 10-20 ohm range if you cannot order a power resistor.

3. ### dl324 AAC Fanatic!

Mar 30, 2015
8,260
1,989
As mentioned, this is meaningless unless you tell us what the nominal panel voltage is.
What is preventing you from putting a load on the panel?

4. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
Current at Pmax (Imp) 1.14A
Voltage at Pmax (Vmp) 17.5V
Short circuit Current 1.28A
Open-Circuit Voltage 22.05V
Maximum system voltage suitable for the panel DC12V

There is nothing preventing me putting a load onto the panel.

I am assuming that with a know fixed load as the sun rises and the voltage increases, then the voltage across the load will be an indication of exactly how much power the panel is putting out.

I am trying to monitor the voltages on cloudy days and full sun days and see the difference. Without a load I am just measuring the voltage across the panel. If there was a load and the voltage is being measured whilst the current is being drawn I will be able to work out the actual power the panel is producing. This is all an assumption on my behalf.

5. ### dl324 AAC Fanatic!

Mar 30, 2015
8,260
1,989
Put a nominal load on the panel and measure the voltage on cloudy/sunny days. That will give you the relative power available for that load under varying amounts of sun light.

6. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
I will put a 12 Ohm Ni-chrome wound resistor across the panel and measure the voltage. Then I will use ohms law to work out the panel. I just want to be clear. The voltage across the load is the one I use in the calculations? Sorry just wanting to be sure

Bernard likes this.
7. ### dl324 AAC Fanatic!

Mar 30, 2015
8,260
1,989
Yes, but you don't need that big of a load; unless that's what the panel will be powering.

8. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
I seem to have got myself totally confused.

I wound a very crude resistor out of Ni chrome wire, the value is 11R2.

I then tested it across a battery the voltage was 14.5 volts. I put the resistor across the battery terminals and the voltage dropped to 13.8 Volts

This is a current draw of 1.23 Amps and power rating of the load 17 Watts.

I then took the same resistor and put it directly across the terminals of the Solar Panel. Before the load the solar panel was 18.57 volts.

As soon as I put the resistor across the terminals the voltage dropped to 1.9 Volts.

Does this mean that my solar panel is only generating 0.33 Watts?

Unfortunately it is very overcast today so no sun to check if that makes a difference.

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
Crystalline type panels need full sun. The fall off in voltage is very steepee and they are almost useless at 50% illumination levels

10. ### KeepItSimpleStupid AAC Fanatic!

Mar 4, 2014
3,254
580
Any load that shows a voltage from 0V to Vmp would be fine. The curve is relatively flat until it's close to Vmp. You really want to measure short circuit current, so the closer to 0V, the better. Open circuit voltage tells you nearly nothing.

11. ### crutschow Expert

Mar 14, 2008
22,016
6,367
To determine the maximum power output you need to match the load to the panel's internal impedance.(MPPT converters do this automatically)
You can do that by adjusting the load resistance until the voltage across the load is ≈1/2 the panel open circuit voltage for the same illumination.
This resistor value will vary with the illumination level.

Alternately you can measure the short circuit current and the open circuit voltage.
The maximum power delivered to the optimum matched load resistance would be 1/4 the open circuit voltage times the short circuit current.

Last edited: Jan 15, 2017
12. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
Measure the resistance immediately after a test.

The current will heat the wire and resistance will increase
Use this HOT resistance value when calculating power.

13. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
Thank you for all the advice. I am totally confused. I dont understand all the terms and technology. I am just trying to find a method to measure what power my panel is producing during various times of the day in different c]kinds of weather

14. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
Maybe a short 4 pages of reading might help

• ###### Solar Cell.pdf
File size:
52.2 KB
Views:
12
RodneyB likes this.
15. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
Thank you, this information sure did help.

On page 4 of the attached document it discusses the Variable load method.

This is as I understand it is to draw an IV curve. the variable load and measuring of the power would require quite specialized equipment.

As I am wanting to just measure the output power of the panel under various conditions I made a fixed load of 5R

I then placed the load across the panel terminals for 2 seconds. I measured the voltage and the current my results were as follows

Open circuit voltage = 17.1 Volts
Voltage on load across terminals of solar panel = 0.56 Volts
Current being drawn = 0.0684 Amps

Using ohms law P=IV
= 0.0684 x 0.56
= 0.0383 Watts

So if my understanding is correct at that point the solar panel was only producing 0.0383 Watts.

It is very overcast and raining today.

If this is correct I can set my equipment to automatically switch the load and measure the voltage then knowing the resistance I can work out the power over the course of the day taken in 5 minute intervals.

This is all assuming that my above assumption is correct.

16. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
Mono crystalline and poly crystalline silicon solar cells need FULL sunlight to produce usable power output.

Amorphous silicon solar panels are slightly less efficient but can produce usable amounts of power in overcast weather conditions.

17. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16
I am seeing that first hand. Are my calculations and assumptions correct?

18. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
Overcast rainy day.
Less than 1 watt from a 20 watt panel

I'd say you are probably right on those numbers.

RodneyB likes this.
19. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
697
16

Thank you so much for your help and all the information.

20. ### Bernard Expert

Aug 7, 2008
5,093
584
Put your 11R2 ohm resistor in a bucket of water ( 13.6 ohms would be better) for a reasonably