How to reduce solar panel VOC (Important!)

The VOC is the Open Circuit Voltage – is your solar panel or a solar array is producing too many volts? If so, there is a simple way to reduce the number of volts that a solar panel sends down the circuit.

But, first, let’s discuss why a solar array may produce too many volts. 

It is the job of the array to produce as much energy as possible. However, there are situations where the array will produce too many volts.

Primarily that is a situation when you have too many solar panels connected to a low voltage controller or other devices. 

In this blog, we discuss:

  • What VOC is to solar energy
  • What Solar Controllers Do
  • The information you need to collect to use a VOC calculater
  • Options for reducing solar output

Solar controllers are rated by the maximum number of volts they can handle. The danger of sending too much voltage to a controller is an electrical fire and damage to other solar components, especially solar batteries. 

Voltage meter checking AC volts white background

What is VOC in a solar cell?

What is VOC? VOC is the maximum voltage of an open circuit produced by a solar panel. Open Circuit Voltage (VOC) and is a product of the forward biases of the solar cell. 

You cannot go by the volts rating on the solar panel box because a 12v solar panel will produce as much as 18v-22v. However, you can use a voltmeter to test the actual voltage.

How many volts the solar panel gives off reflects how many cells the solar panel has and the rating for voltage per cell. 

How can you reduce the voltage of a solar panel?

The first thing to do is double-check your calculations before you buy solar panels and your solar regulator. Your goal is to keep the voltage from the panels at 2/3s of the average maxim voltage of the controller.

For example, if the controller is rated at 150 volts, you want to keep the average solar output to the controller around 100 volts.

Doing so takes into account the varying amount of energy a solar panel produces throughout a day. For example, in the morning, the energy will be lower than it is at noon. 

See also: Calculate Solar Panel kWp & KWh (KWh Vs. KWp + Meanings)

Keep The String Short

The second part is fully controllable. Most of the trouble from overloading a solar controller occurs when you string too many solar panels together.

For example, if your solar controller has a maximum voltage of 150 volts, and each of your solar panels produces 36 volts, and you string five panels into a string, you get 180 volts.

The answer is to string fewer panels; in this example, strings of three panels are ideal. If you need to string five 36volt panels together, you need a controller with a maximum voltage rating in the 270-volt range. 

The answer is to adjust the number of panels or to increase the capacity of the solar controller. There are other possible answers, such as creating circuits that reduce energy is just wasting potential energy.

You’ve paid for the panels; use as much of the energy as possible. Doing so raises the ROI of the investment you’ve made in the panels. 

Other Options

If you are grid-tied, other options would be to allow the excess energy to flow to the utility grid. That is not a bad solution if your local utility provides you with solar credits. If they don’t, you are giving away energy for free.

There is the small benefit of providing clean energy where other consumers might still use dirty energy. 

What happens if you are off-grid? If your solar array is producing too much energy, you have two choices. Reduce the number of panels or find a way to use more energy.

Off-grid systems have battery backup, and if there is too much energy passing through the control, those batteries will die prematurely.

You can install fuses and breakers before the solar controller, but you must constantly monitor the array. 

What is VOC in solar charge controllers?

A VOC solar charge controller is a device that limits the amount of energy that passes through it. We often see these in solar array systems where a solar battery storage system is in place.

They are sometimes called step-down controllers or Bucks Controllers. Other types of solar controllers include PWM and MPPT.

All of these work to regulate the amount of voltage coming from the array. Each is available in different ratings for maximum voltage. 

The other solar panels’ other job is to stop the battery storage system from reverse feeding energy back into the panel.

So when the sun goes down, and the chemical reaction within the panels stops, the energy from the batteries or the grid, if you are grid-tied, can flow backward into the solar array. The solar controller prevents that from happening. 

Solar Controller Sizing 

Solar controllers come in a range of sizes, and you match them to the output of your solar array. As described above, a good rule of thumb is to choose a solar controller that can handle more energy than your array can produce.

That tip helps compensate for the fact that the amount of energy a solar panel creates is not static; It is dynamic and will ebb and wane throughout the day. 

How do you calculate the Voc of a solar panel?

Calculating the VOC of solar panels is complicated. Thankfully, there is a VOC Calculator

What you will need to know is:

  1. The Solar Panel Open Circuit Voltage (VOC)
  2. Solar Panel Maximum Power Point Voltage (Vmp)
  3. Solar Panel Temperature Coefficient of Pmpp
  4. Solar Panel Temperature Coefficient of VOC. 

If your eyes are rolling back in your head, you can relax. All of this information is on the solar panel data sheet that is attached to your solar panel. Chances are you can also find all of this information online at the manufacture’s website.

Additional Data Needed 

5. is the maximum temperature for the site, in Celsius. 

6. What is the minimum temperature for the site in Celsius. 

7. How many Solar Panels are in a string. 

Once you plug in all of this data, the calculator will do the rest for you. 


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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