How to interpret solar panel specifications and use them to choose the proper panels for your application.
On a solar panel, module, cell, or laminate, there are a number of different specifications given. Here’s what you will find, and how to interpret it.
Here are some sample specs I, uh… stole for purposes of illustration. These are for a Canadian Solar CS6P-190-PE.
Power (W): 190 Watts
Open Circuit Voltage (V): 36.00 Voc
Short Circuit Current (A): 7.42 Isc
Maximum Power Voltage (V): 28.60 Vmp
Maximum Power Current (A): 6.64 Imp
Now, here’s what these mean to you!
First off, the Open Circuit Voltage (Voc). This is the voltage you will see present at the solar panel’s output when it is exposed to full sun and is not loaded. While this is not really relevant to the panel’s power output, it should be taken into consideration for two reasons: First, you should ensure that any equipment connected to the panel (meters, charge controllers, etc) is capable of handling the full Voc of the solar panel or the string of solar panels connected. Otherwise, equipment damage may occur when the sun hits the panels and they’re not loaded down. Consult the documentation on your charge controller if in doubt. Also, for your safety, be sure that any overcurrent protection devices or disconnect switches are rated for Voc or higher! Upon unpacking and installing your panels, if you’re lucky enough to get full sunlight hitting them, check the open circuit voltage – it should be close to Voc. If it’s too low, the panel may have a problem or be miswired (check the junction box).
Short Circuit Current (Isc). This measurement is useful for testing the panels and determining the sizing of your wiring and controller. Set your multimeter to amps, and connect the leads across the solar panel’s output terminals. In full sun, you should get Isc. If you do, the panel is operating correctly. Any wiring to the solar panels, and the charge controller itself, must be capable of handling the Isc of the array. Do not expect to load the panel down to Isc in normal operation, as you will be getting almost no voltage and extremely reduced power. (See I-V curve below!)
Maximum power voltage (Vmp) and amperage (Imp). These levels are very important to consider in selecting panels and components for your solar energy system! In short, please keep the voltage as close to Vmp as possible. The reason for this is that the solar panel has a certain internal impedance, and you will only receive maximum power when the panel output voltage *under load* is allowed to remain near Vmp. If you load the panel down to a lower voltage, it will become severely inefficient.
If you are using the solar panel with a conventional charge controller to charge lead-acid batteries, the ideal Vmp will be near the absorption charge voltage for your batteries. If you are using an MPPT charge controller, Vmp should be anywhere within the controller’s MPPT tracking range. This may be an extremely wide range of voltage, allowing you a lot of flexibility in choosing panels!
To illustrate the importance of the Vmp point, see the above I-V curve and power curve for a solar panel. Note that the power curve tapers down towards zero as the voltage falls below Vmp, and abruptly falls off as the voltage approaches Voc. If you buy solar panels and operate them too far from Vmp, you might as well be throwing money down a hole.
If you have an MPPT charge controller, it will periodically sweep the array voltage to find Vmp, which actually varies a little with different sunlight levels. For the best possible power output under all conditions, use an MPPT controller. The only exception I should point out: if you are using HF radio equipment, the switchmode boost/buck converter inside an MPPT controller may cause excessive noise on the receiver. Consider using a simpler charge controller in this case. The Morningstar ProStar series charge controllers have an internal jumper (really, a 0 ohm resistor that you can cut) to disable their PWM charge control to reduce RF noise to minimum.
Don’t toss your system efficiency and money down the toilet – choose your solar panels wisely!
21 thoughts on “Solar Panel Specifications Explained”
Are all of these articles written yourself or did you hire a ghost writer?
It’s just me. Sorry for the late reply, akismet tried to eat your comment. Sometimes it gets a little too hungry
Hi, Nice and practical information you collected! Good work!
Can you explain this statement in a little more detail:
“Do not expect to load the panel down to Isc in normal operation, as you will be getting almost no voltage and extremely reduced power. (See I-V curve below!)”
What do you want to say here and its relation with the I-V Curve!
Best Regards. Vicky
Wonder if you (or someone) could explain something I don’t seem to be understanding.
1. Re: battery charging applications ( RV !) ….
A battery is charged by current flowing through it in reverse, thus reversing the chemical reaction that produces the battery output when discharging. Although you do need voltage to cause a current to flow, it is the CURRENT flowing through the battery that charges it.
When you connect a battery to a solar panel, say in full sun, the panel voltage is pulled down to the battery voltage which could be, say, 12.4 volts. When I read the IV chart above for a solar panel, it pretty much puts out the same current up until you get close to the maximum power point. It won’t be putting out the maximum POWER ( Watts ) but it will still be putting out close to the maximum current.
So, then, why would you need to run it close to the maximum power point?
I guess what I am trying to say is you dont charge a battery with WATTS ( power ). You charge it with AMPS( current ). Is this completely wrong headed?
I understand exactly what you’re asking and I struggle with it too. Anybody able to find the words to illuminate us?
MPPT magic takes the extra voltage and changes it into more current at a lower voltage. Panels run at Vmp and something close to Isc, once the MPPTs buck converter does it’s stuff the battery is presented with it’s ideal charging voltage which is lower than Vmp. However, that difference in voltage between panel Vmp and battery happy voltage, is converted to extra current. So battery gets more current than basic panel Isc. I THINK!
Hi , i am a little confused , can anyone tell me for a 150 vdc mppt charger what i use to calculate the max vdc . the open circuit(VOC)-37,74V or the VMP-30,03V? Thank you !
Use Voc to calculate maximum voltage for a controller.
If your average monthly bill is about $160 before you get solar panels on your house, a typical 5-kW solar system might save you about half, depending on where in the country you live.
Thanks a lot for these very useful and well written infos
Please what is the relationship between the Panel Specifications and the I-V Curve.
On the I/V curve, Isc appears at the 0 volt end, Voc is at 0 amps, and Vmp is near the knee.
Can I connect the output of two 6vdc panels in series to get a 12v output???
I came here looking for vmp info. Now I confused myself even more… I am thinking of running 4 100w 12v panels in a 2p2s configuration. I am thinking that the total system would be 400w, 24v and 17amp. I was looking at the ROV20 charge controller but some of the numbers on the front are making me second guess myself. Specifically the solar Vmp for 24v is 36-75v. Would this charge controller be viable for me setup?
Connect all your panels in series and obtain Voc of 48V. This is within the charger limits (36-75V). This installation is cheaper, because you can use a thinner wire. Higher voltage and less current for the same power.
What is Open circut voltage
That’s the voltage across the panel in full sun with no load
between an Isc and Imp which is higher
Isc will be higher than Imp. Please keep in mind though that Isc is just the amount of current that will flow when you have loaded the panel down to almost zero volts… it’s really just there for safety considerations in sizing wires and fuses.