Solar Panel VMP vs VOC (Important Practice)

You’ll find a specifications sheet when you unpack your solar panel package. This sheet will contain 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 can tell what loads will require and 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
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 refers to measuring how many volts the solar panel generates with no inverter, charge controller, batteries, or anything else connected. You will obtain an accurate VOC reading if you measure your leads with a voltmeter while your solar panel isn’t connected to any other equipment.

Perform this test at different times of the day. The average reading will give you a good idea of the maximum voltage 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 produce. You need to know that number to 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.

Sun Beating Down On Solar Panels
Sun Beating Down On Solar Panels


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 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.

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.

Electrical Breaker Box. - Sol Voltaics: Solar Panel Information
Electrical Breaker Box.

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 works 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. Ensure they’re both within the “maximum power point tracking circuit” range 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.

Ensure 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 ensure the numbers align with your solar panels’ Voc and Vmp figures.

Picture Of Solar Panels In The Evening On Sunset. - Sol Voltaics: Solar Panel Information
Picture Of Solar Panels In The Evening On Sunset.

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 your charge controller can endure without damage.

Wind 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.

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

Solar Regulator Charge Controller Mttp
Solar Regulator Charge Controller Mttp

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.

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 use two or more panels.

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

Remember 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%.



General Solar Panel FAQ

What Problems Do Solar Panels Solve?

In environmental terms, solar panels can potentially solve a handful of problems, including;
1. Air pollution
2. Water pollution
3. Greenhouse gases
4. Reduction in fossil fuel use

For individuals, solar energy allows you to become completely self-sufficient when it comes to your electricity needs and can save you a lot of money in the long run.

What Are 3 Important Uses Of Solar Panels?

The three most important uses of solar panels are;
1. Solar electricity. This can be used to power almost any appliance in your home, including TVs, computers, and fridges.
2. Lighting. In addition to the use of low-power, LED lightbulbs, solar panels can provide an efficient, low-cost, and environmentally friendly way to provide lighting to homes. 
3. Portable solar. In our modern, always-connected lives, our phones, tablets, and computers are almost always with us, and all run on batteries. Portable PV chargers can help keep our batteries topped up no matter where we are, as long as there is some sun to charge them.

Do solar panels give you free electricity?

Once the cost of the array is paid in full, the energy it produces is free. There are ongoing maintenance costs, too, such as annual panel cleaning, etc. 

How much will my electric bill be with solar panels?

Suppose your solar array includes a solar battery backup system, and it is large enough to fully cover your energy usage per day. In that case, your monthly electric bill will be next to zero dollars, even with a grid-tied system. 

If your solar array does not include a solar battery backup system, then at night, your house or business will use grid electricity. That cost will vary but expect to pay from 1/3-2/3 of your average electric bill, and that cost will fluctuate seasonally. 

Do you save money with solar panels?

The simple answer is, Yes, you save money with solar panels. There is an initial upfront cost, but since solar panels are warrantied for 25 years, you will save money over time. You will also begin to see monthly savings in energy bills, but there are other ways that solar panels pay you back. Those include:
1. Adding value to your home or commercial building 
2. Monthly decreases in energy costs
3. The ability to add more energy appliances without increased monthly costs
4. The potential for tax credits for going solar

Can solar panels power a house 24-7?

Most definitely! Solar panels can certainly power a house 24-7, with the addition of a high-quality inverter and a suitable battery bank, of course. To power, a house under normal usage will require a massive solar array, though, and there will be a very expensive initial financial outlay.

Do I need to tell my energy supplier I have solar panels?

This depends on where you live, but in most cases, it’s not necessary to inform your energy supplier that you have solar panels. That said, you may be producing excess power with your solar system, in which case you may be able to sell that excess power back to energy companies.

In this case, you’ll naturally need to be in contact with them. 

What Are Solar Cells Known as and Why?

Solar cells are also called photovoltaic (PV) cells. They are called so because the term ‘photovoltaic’ literally means light i.e. photo and electricity i.e. voltaic.

These cells generate electricity through the photovoltaic effect. This effect basically causes the generation of free electrons from the semiconducting silicon material of the solar panel when sunlight hits its surface.

What Type of Solar Panels Are Most Efficient and Why?

There are currently three types of solar panels available in the market that are:
1. Monocrystalline
2. Polycrystalline
3. Thin-filmed

Among these, monocrystalline solar panels are known to be the most efficient among all others.

Does heat enter your home through the roof?

Absolutely. Heat enters your home through your roof, and on a hot day your attic can get up to 150 degrees Fahrenheit or more. Through conduction, heat from the sun warms your roof which then warms your attic and the rest of your home.

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