# Solar Panel Voltage Calculator

**Note:**Based on your inputs, this charge controller has a suitable maximum PV voltage for your solar array. However, it may not be the right option for your setup based on other factors such as current rating and battery bank voltage, so check that it meets all your other requirements before going with this option.

## What Is Solar Panel Voc?

Solar panel Voc is short for solar panel open circuit voltage. It is the maximum voltage of a solar panel when it isn’t connected to any load – no charge controllers, inverters, or anything.

All solar panels come with an open circuit voltage rating. However, this rating is based on results obtained under standard test conditions.

Those conditions are a 25℃ solar cell temperature, air mass of 1.5, and solar irradiance of 1000 W/m².

While the rated maximum open circuit voltage gives an idea of what to expect, it does not always reflect reality. In reality, ambient/operating temperature significantly affects open circuit voltage.

With higher ambient temperature comes increased cell temperature. Then increased cell temperature causes open circuit voltage (Voc) to decrease.

Conversely, decreased ambient temperature means lower cell temperature. With lower cell temperature comes increased Voc.

This is why you might just get a pure Voc reading if you try to measure it early in the morning – just around when the sun starts hitting the panels.

## Why Calculating Maximum Open Circuit Voltage Is Vital

Every solar system unit, from the inverter to the charge controller and battery, operates within a specific voltage range. Going above that range will spell disaster for the said units.

So, to ensure that the voltage running through the charge controller, battery, inverter, and other solar system units is within safe limits, we have to calculate the maximum open circuit voltage.

Open circuit voltage is calculated using solar panel temperature coefficient and ambient temperature.

When we know solar panels temperature coefficient and the lowest temperature to expect at the site, we can readily estimate the maximum open circuit voltage.

## Solar Panel Maximum Power Point Voltage (Vmpp)

A solar panel’s maximum power point voltage (Vmpp) is the voltage of the solar panel at peak power output. Unlike Voc, it is measured when loads (charge controllers and inverters) are connected to the panel.

Like Voc, operating temperature significantly affects Vmpp. Vmpp also varies throughout the day and changes with weather and climatic conditions.

Operating temperature significantly affects Voc and Vmpp in the same way. So, it’s no surprise Vmpp is typically lower at a higher cell temperature. Then at a lower cell temperature, Vmpp would be higher.

### Reason to Estimate Vmpp

Vmpp simply gives an idea of the solar panels’ max voltage (max power output). It is more of a performance metric.

Voc, on the other hand, is more of a safety metric. It tells us the maximum voltage our solar array will produce to ensure the loads (charge controllers and inverters) can safely operate at that voltage.

## How to Use the Solar Panel Voltage Calculator

**Enter your solar panels’ open circuit voltage**in the “Open circuit voltage (Voc)” field. You can find this information in the solar panel datasheet or product manual.- If the panels have the same specifications,
**enter how many solar panels you connect in series**in the “Quantity” input field. But if the panels have different specifications, click on the “+ Add a Panel” button below the “Temperature coefficient of Voc” field to add each panel. **Enter your solar panel Voc temperature coefficient**in the “Temperature coefficient of Voc” input field. Then click on the options on the right to select the appropriate temperature coefficient unit. There are two options: %/℃ and mV/℃.**This step is optional. If you do not specify a temperature coefficient, the solar panel voltage calculator will assume a correction factor based on the lowest temperature you expect around your solar panels.****Enter the expected minimum temperature the solar array will experience at the installation site**– this could be the lowest recorded temperature in your location. Then choose the corresponding temperature unit from the options on the right.- After filling in all the compulsory fields,
**click on the Calculate button**. After clicking the button, the solar panel voltage calculator will display your maximum open circuit voltage. It also recommends a charge controller for your solar array based on the maximum open circuit voltage.

## How to Calculate Solar Panel Maximum Open Circuit Voltage (Voc)

A solar panel voltage calculator is not the only way to calculate open circuit voltage. You can also estimate it using any of the following methods:

- Temperature coefficient of Voc
- Correction Factors

### How to Calculate Voc Using Temperature Coefficient

This method does something similar to the solar panel voltage calculator.

To estimate the Voc of a solar array using the temperature coefficient of Voc, we need the following:

- The Voc of all the solar panels in the solar system
- Number of solar panels in series (if the panels are identical)
- Lowest expected temperature
- Solar panels temperature coefficient of Voc (we can get this from the solar panel datasheet)

Once we have the values of all the four quantities above, we can estimate Voc following these steps:

#### Calculate the Maximum Temperature Difference

The maximum temperature difference is between the standard test temperature and the lowest temperature at the solar panel site.

We’ll simply subtract 25℃ from the lowest expected temperature to estimate the maximum temperature difference. As mentioned earlier, we subtract from 25℃ because that is the standard test temperature.

\(maximum\ temperature\ difference = lowest\ expected\ temperature\ – 25℃\)

#### Calculate the Percentage Increase of Maximum Voltage

After calculating the maximum temperature difference, we’ll use its value to estimate the percentage increase of the maximum voltage.

The percentage increase of the maximum voltage of each solar panel estimates how much the open circuit voltage is expected to shoot up.

We can estimate its value by multiplying the maximum temperature difference by the solar panel temperature coefficient of Voc.

\( \begin{gather} percentage\ increase\ of\ maximum\ voltage = \\

maximum\ temp\ differential * temperature\ coefficient\ of\ Voc \end{gather}\)

When the solar panels are not identical, we’ll have to do the calculation for each solar panel. But if the panels are similar, we’ll multiply by the number of panels.

#### Calculate the Maximum Open Circuit Voltage of Each Solar Panel in the Solar Array

Once we know the percentage increase of the maximum voltage, we can calculate the actual maximum open circuit voltage.

To calculate the maximum open circuit voltage of each solar panel in the solar system, we’ll use the following formula:

\(\begin{gather} maximum\ open\ circuit\ voltage = \\

open\ circuit\ voltage * (1 + \displaystyle {\frac {percentage\ increase\ of\ maximum\ voltage}{100}}) \end{gather}\)

*open circuit voltage here refers to the open circuit voltage stated on the solar panel datasheet.*

As said in the previous section, when the solar panels are not identical, we’ll do multiple calculations using the open circuit voltage on each solar panel datasheet.

#### Get the Total Maximum Voc of the Solar Array

After calculating each maximum solar panel Voc, we can sum them up to get the total maximum Voc of the solar system.

If the solar panels are identical, we can just multiply the max solar panel Voc by the number of solar panels in series:

\(total\ maximum\ Voc = max\ solar\ panel\ Voc * number\ of\ solar\ panels\ in\ series\)

For unidentical panels, we’ll have to add the max solar panel Voc of each panel:

\(\begin{gather} total\ Voc\ = max\ solar\ panel\ Voc\ of\ panel\ 1 + max\ solar\ panel\ Voc\ of\ panel\ 2\ + \\

max\ solar\ panel\ Voc\ of\ panel\ 3 + … + max\ solar\ panel\ Voc\ of\ panel\ n \end{gather}\)

#### Example 1

An array of 5 identical solar panels in series have the following specifications:

- Temperature coefficient of Voc is -0.5%/℃
- Voc of each solar panel is 23.3V
- Lowest recorded/expected minimum temperature is -8℃

What is the maximum Voc of the solar system?

First, we’ll work out the maximum temperature differential:

\(max\ temperature\ differential = -8 – 25 = -33℃\)

Next, we’ll the percentage voltage difference (percent increase of Voc):

\(percentage\ increase\ of\ maximum\ Voc = -33 * -0.5 = 16.5\%\)

Now that we know the percentage voltage difference, we can work out the maximum Voc for each solar panel:

\(max\ open\ circuit\ voltage = 23.3 * (1 + \displaystyle {\frac {16.5}{100}}) = 23.3 * 1.165 = 27.1445V\)

Finally, we’ll work out the max open circuit voltage of the system. Since the solar panels are identical, we’ll multiply the maximum Voc by the number of panels:

\(Total\ Voc\ of\ the\ system = 27.1445 * 5 \approx 135.72V\)

#### Example 2

An array of 3 solar panels in series have the following specifications:

- Temperature coefficient of Voc is -0.5%/℃, -0.3%/℃, and -0.4%/℃
- Voc of each solar panel is 20.3V, 22.6V, and 21.8V
- Lowest recorded temperature/expected minimum temperature is 1℃

What is the maximum Voc of the solar system?

First, we’ll estimate the max temperature differential:

\(max\ temp\ differential = 1 – 25 = -24℃\)

Next, we’ll estimate the percentage increase of Voc for each panel.

For panel 1, the percentage Voc increase would be:

\(percentage\ Voc\ increase = -24 * -0.5 = 12\%\)

For panel 2:

\(percentage\ Voc\ increase = -24 * -0.3 = 7.2\%\)

For panel 3:

\(percentage\ Voc\ increase = -24 * -0.4 = 9.6\%\)

Now, we’ll work out the maximum Voc for each panel.

For panel 1, the maximum Voc would be:

\(maximum\ Voc = 20.3 * (1 + \displaystyle {\frac {12}{100}}) = 20.3 * 1.12 = 22.736V\)

For panel 2:

\(maximum\ Voc = 22.6 * (1 + \displaystyle {\frac {7.2}{100}}) = 22.6 * 1.12 = 24.227V\)

For panel 3:

\(maximum\ Voc = 21.8 * (1 + \displaystyle {\frac {9.6}{100}}) = 21.8 * 1.096 = 23.893V\)

Finally, we’ll work out the maximum Voc of the system by adding the individual maximum Voc of the panels:

\(total\ Voc\ of\ the\ system = 22.736 + 24.227 + 23.893 \approx 70.86V\)

### How to Calculate Voc Using Correction Factors

Another way to work out Voc for monocrystalline and polycrystalline PV panels is to use the correction factors provided by the National Electric Code (NEC):

To estimate the Voc of a solar array using correction factors, we need to determine the following:

- The Voc of the panels in the solar system
- Lowest expected temperature

#### Calculate the Maximum Open Circuit Voltage of Each Solar Panel in the Solar Array

To estimate the maximum Voc, multiply the solar panel voltage by the correction factor corresponding to the lowest expected temperature:

\(maximum\ Voc = solar\ panel\ voltage\ (Voc) * correction\ factor\)

If the solar panels have the same Voc, then this one calculation should do. But if the panels have different open circuit voltages, we’ll work out the maximum Voc for each panel in the solar system.

#### Get the Total Open Circuit Voltage (Voc) of the Solar Array

After calculating the maximum Voc for each panel, we’ll sum them up to get the total Voc of the system.

For identical panels:

\(total\ Voc = maximum\ Voc * number\ of\ panels\)

For unidentical panels:

\(\begin{gather} total\ Voc\ = maximum\ Voc\ of\ panel\ 1 + maximum\ Voc\ of\ panel\ 2\ + \\

… + maximum\ Voc\ of\ panel\ n \end{gather}\)

#### Example 1

An array of 4 identical solar panels in series have the following specifications:

- Voc of each solar panel is 22.7V
- Lowest expected ambient temperature is -5℃

What is the max open circuit voltage of the solar system?

Since the lowest ambient temperature is -5℃, we’ll use 1.12 as the correction factor (see table above)

First, we’ll work out the maximum Voc for each panel:

\(maximum\ Voc = 22.7 * 1.12 = 25.424V\)

Then, we’ll estimate the maximum Voc of the whole system. Since the panels are identical, we’ll multiply the maximum Voc of one panel by the number of panels in the system:

\(maximum\ Voc\ of\ the\ system = 25.424 * 4 \approx 101.7V\)

#### Example 2

An array of 2 solar panels in series have the following specifications:

- Voc of each solar panel is 25.3V and 24.6V
- Lowest expected ambient temperature is 3℃

What is the max open circuit voltage of the solar system?

Since the lowest expected temp is 3℃, our correction factor would be 1.10.

We’ll start by calculating the maximum Voc for each panel:

\(maximum\ Voc\ for\ panel\ 1 = 25.3 * 1.10 = 27.83V\)

\(maximum\ Voc\ for\ panel\ 2 = 24.6 * 1.10 = 27.06V\)

Next, we’ll add the maximum Voc for each panel together to get the maximum Voc of the whole system:

\(maximum\ Voc\ of\ the\ system = 27.83 + 27.06 = 54.89V\)

## Calculating Open Circuit Voltage (Voc) for Solar Panels in Parallel

When solar panels are connected in parallel, the maximum Voc of the connection would equal the maximum Voc of one of the panels.

In other words, if we connected two solar panels whose maximum Voc is 23.3V, the maximum Voc of the solar array would be 23.3V.

But then, if the solar panels in parallel do not have the same maximum Voc, you could use the lower/lowest maximum Voc of the panels as the system’s maximum Voc.

## Calculating Voc in Series-Parallel Connection

If the panels are in a series-parallel connection (multiple series strings connected in parallel), we’ll estimate the maximum Voc of each series string.

Then we’ll use the lowest maximum Voc amongst the strings as the array’s maximum Voc.

## Choosing Charge Controllers Using Max Solar Panel Voltage

As we mentioned, if your solar panel’s output voltage exceeds your charge controller’s limit, there might be a problem.

To avoid that problem, we have to choose a charge controller using the maximum Voc of the solar system.

To choose a charge controller using maximum Voc, compare the maximum Voc with the maximum input voltage of the charge controller.

The max Voc of the solar array **must not be higher** than the maximum input voltage of the charge controller. If it is, you’ll need to get another charge controller whose maximum input voltage is higher than the system’s max Voc.

The max input voltage of your charge controller may also be written as maximum PV voltage or maximum PV open circuit voltage.