Solar Panel VMP vs VOC (Important Practice)

When you unpack your solar panel package, you’ll find a specifications sheet. This sheet will be loaded with lists of abbreviations and numbers, including VOC and VMP. This can be pretty confusing. What does it all signify?

Once you know how to read a spec sheet, you’ll be able to tell what kinds of loads will require how many solar panels, batteries, and charge controllers by studying the ratings.

I’ll be delving into the nature of VOC and VMP in this discussion. It’ll include the following points:

• Definitions of Voc and Vmp
• How do these ratings differ from one another
• Which one is most important to consider when buying a solar panel
• Relation of ratings to charge controllers and batteries
• Relation of ratings to weather conditions

It’s surprising how many factors go into a well-considered purchase of solar panels. How do you go about learning about Voc and Vmp? Read on if you want to learn more.

Voltage meter in a plug socket

What Are VOC and VMP Ratings on Solar Panels?

VOC means Voltage at Open Circuit and Vmp refers to Voltage at Maximum Power. What do these terms refer to?

VOC

VOC refers to the measurement of how many volts the solar panel generates with no inverter, charge controller, batteries, or anything else connected to it. If you measure your leads with a voltmeter while your solar panel isn’t connected to any other piece of equipment, you will obtain an accurate VOC reading.

Perform this test at different times of the day. The average reading will give you a good idea of the maximum voltage that your solar panel can produce. If you’re interested in using multiple solar panels, this figure will tell you how many panels you can wire into your inverter or charge controller without overloading them.

A pure Voc number can be produced for a few moments in the morning, at least in principle, when the sun’s rays just begin striking your panels.

This is when they’re at their coolest for the day. Your electronics will not yet be active. But at other times during the day, Voc will never be produced cleanly.

The Voc number for your panel will give you the optimum voltage it will ever produce. You need to know that number, so you can set your system safely. Avoid overpowering your electronics with too much voltage.

This could damage them. Fuses and breakers are good protection against over-current, but not over-voltage.

Vmp

On the other hand, with Vmp, you must connect your panel to an inverter or charge controller to determine what voltage the panel will produce. Vmp is measured when you connect your solar panel to a load.

Measure Vmp with a multimeter at the input terminals of your charge controller. You’ll soon note that your panel’s Vmp will usually measure out to be about 70% to 80% of its Voc.

Proper measurement of Vmp will occur when the power output is the greatest when your panel is performing at its peak. This usually occurs around sunrise when the panel is at its coolest temperature. The sun’s rays are just starting to hit the panel at that time.

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Vmp will then vary throughout the day depending on temperature, clouds, obstructions, shade, and dirt.

Make sure the panels are connected to a load when you run this test. Of course, as the load or battery draws down the Vmp during the day, the measured voltage will drop below the optimum Vmp.

What’s the Difference Between Voc and Vmp Regarding Your Solar Panel’s Output?

VOC will give you information on the number of solar panels you’ll need to power your electronics.

Vmp will give you the maximum voltage your solar panels will generate under ideal conditions.

Which One is More Important for Solar Panel Voltage?

VOC is an ideal number. It is ordinarily never reached during normal operations. Vmp is an operational number, achieved as the solar panel is doing its work while connected to a load.

Both measurements are important, but I’d say Vmp is more important.

How Do Voc and Vmp Ratings Relate to Charge Controllers?

Your solar charge controller or inverter must withstand both Voc and Vmp during the day. Make sure they’re both within range of the “maximum power point tracking circuit” in the charge controller or inverter.

All charge controllers have a maximum input voltage. You must make sure your solar panels will never exceed this voltage.

Make sure the Voc ratings of your solar panel never exceed the maximum input voltage of your controller. The controller might not have time to dampen the voltage in the morning as the solar panel starts generating electricity rapidly.

Reflected light from snow on the ground could make it worse. A clear, cold day could fry your controller.

Err on the side of safety. Make sure the Voc of the panel is well below the maximum for the charge controller—by roughly 20%.

If you’re using more than one solar panel in parallel, the voltage will remain the same, but you must multiply the current by the number of panels. Bear this in mind while studying your charge controller’s maximum current ratings.

Study your charge controller’s spec sheet to make sure the numbers align with your solar panels’ Voc and Vmp figures.

How Does Weather Affect Solar Panel Energy Production?

Solar panels convert the sun’s rays into electricity, but solar heating of the panels, ironically enough, acts to make the panels less efficient at energy production. Voltage production will drop 0.33% for every degree above 25 C.

Winter can also introduce unexpected challenges to your solar production and conversion systems. Under the combination of sun, cold, and brilliant snow glare, Voc can exceed the maximum voltage that your charge controller can endure without damage.

Wind, on the other hand, can be a surprising help to solar panel energy production. Wind will cool your solar panels, making them far more efficient. Your panels’ efficiency may approach the ideal Voc rating on a blustery day.

Make sure you check whether ratings on your solar panel’s spec sheet. Err on the side of caution, especially if you live in a cold climate.

How to Approach your Vmp Optimum without Endangering Your Electronic Equipment

Consider purchasing a Maximum Power Point Tracker (MPPT) instead of an ordinary PWM charge controller. The PMW must drop panel voltage to match battery voltage, drastically reducing panel efficiency.

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The MPPT will allow the solar panel to operate at maximum, making it 30% more efficient, depending on the batteries and the panel’s Vmp rating. I recommend using the MPPT charge controller if you’re using two or more panels.

You may need to oversize your solar array so that it can produce enough power during bad weather and high temperatures.

Keep in mind the problems introduced by dust and dirt, shading, ordinary cloudiness, and decline in solar panel efficiency over time. Losses can grow to be as high as 20%. A good MPPT charge controller can handle oversizing by as much as 150%.

Sources

  • https://www.fluke.com/en-us/learn/blog/electrical/what-is-ohms-law
  • https://forum.solar-electric.com/discussion/5001/glossary
  • https://www.electronicsforu.com/market-verticals/solar/difference-nominal-voltage-voc-vmp-isc-imp-solar-panels
  • https://www.solarpaneltalk.com/forum/solar-panels-for-home/solar-panels-for-your-home/348775-voc-or-vmpp
  • https://www.solar4rvs.com.au/buying/buyer-guides/how-to-read-solar-panel-specifications/
  • https://diysolarforum.com/threads/vmp-or-voc-for-charge-controller-sizing.10305/
  • https://www.imnotlostimrving.com/solar-panels-charge-controllers/

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