# Solar Panel System Guidelines

Renewable and Sustainable Energy with Solar Panel System

Chapter 6

## Solar Panels String Sizing

Designing a solar panel system by yourself is a difficult task.

While most people prefer buying the systems directly, a few like being more hands-on with their devices. They modify them according to their specifications.

If you prefer being hand-on, string sizing is for you. We will first discuss what string sizing is and then give you a detailed guide to figure out the optimal string size for your panels.

## What Is String Sizing?

A string refers to a group of panels plugged into a single input through a solar inverter.

String sizing are the calculations you need to make to ensure that your inverter receives the exact amount of power required for optimal performance. Since an inverter has a specific range of voltage which it can handle, your string sizing calculations must be correct.

If the inverter receives too little voltage, it won’t have sufficient power to turn on. On the other hand, if it gets too much voltage, you risk damaging your inverter.
Simply ensuring that the provided voltage falls within your inverter’s input range is also not enough. It only gives power to your appliances in that situation.

The purpose of string sizing is to ensure that the voltage falls within a narrower margin called the maximum power point range. At this range, your inverter will operate at its optimum efficiency.

## How To Calculate Solar Panel System String Size Calculating your string size is a slightly complex process that requires analysis of many factors. For simplicity, we will proceed step-by-step. Let’s start with the voltage.

Every solar panel has an output voltage, as mentioned earlier, which goes straight to the inverter. With regards to the output voltage, you should know 2 things:

1. Open-circuit voltage (VOC): This condition occurs when there is no electricity passing through your electrical system, i.e., the circuit is open. The inverter is turned on during this time.

2. Maximum-power voltage (VMP): This is a measure of the voltage that flows in your circuit after you turn on the circuit.

You should find out the value of both VMP and VOC. They are usually available on your panel’s specifications sheet.

The next value you should know is the maximum power point range. It was explained earlier as the sweet spot where your inverter has the highest performance.

One thing you should always know is the maximum direct current input voltage. Remember that your inverter’s circuitry will be overheated and damaged if it exceeds a specified value.

Another thing you should be aware of is the minimum voltage required to start your inverter. However, this isn’t as important, since your inverter functions well above the minimum level anyway.
Okay. You’ve managed to obtain your figures. What next?

### Case Study

Let’s apply the formula to see how it works. We will take a 9-kilowatt grid-tied system as an example.

Suppose our imaginary panels have a VOC of 48.08 and a VMP of 39.28. The maximum voltage for our device will be 360 volts.
Let’s further assume that our imaginary inverter has an MPP range of 270 to 480 volts. Our imaginary inverter should have an operating range of 100 to 600 volts, and its power capacity is 7700 kilowatts.

We have all of the information we need. Let’s begin with a step-by-step calculation.

### 1. Find the Minimum String Size

You will identify the number of panels needed in a string by finding the minimum string size.

You divide the least amount of the MPP range by the VMP of the panel to obtain this value.

The result we obtain is 6.87. Round this to the next whole number. You can’t have a partial solar panel. Right?

### 2. Find the Highest String Size Below the Operating Voltage We find the highest string size which won’t exceed the operating voltage to ensure that we don’t overburden our inverter.

Divide the maximum direct current input by your panel’s VOC to obtain this value. We calculated VOC in the step above.
The answer is 12.48. Remember that you must round up this value since we’re trying to stay within the operating voltage limit.

### 3. Check Whether the String Size Falls within the MPP

We need to be certain that the value we obtained for our string size falls within the inverter’s MPP for optimal functioning.
Divide your maximum string size by your panel’s VMP to obtain this value.

The answer we get is 471.36 volts.

This value falls within our MPP range, as we can see. What are you supposed to do if it is more than your MPP? Decrease the number of panels by one and do the calculation again to see if the new value falls within the desired range.

### 4. Temperature

Finding out the optimal number of panels isn’t enough. The temperature of the area of solar panel system installation play a significant role in determining the MPP conditions.

Find out the average temperature of your locality reliable website. Open that website and find the coldest temperature ever recorded in that area.
Let’s suppose that our imaginary land has the coldest temperature of -33.3 degrees Celsius.

Then, open your solar panels’ specification sheet once more and find out what the temperature coefficient of VOC is. This number tells us the amount of change in the voltage away from the normal operating cell temperature for (NOTC) every unit change in degree.

The temperature coefficient we will take is 0.28%

Let’s also assume we have a NOTC of 44 degrees Celsius.
After that, take the difference between the NOTC and the coldest temperature ever recorded. Our value is 77.3 degrees Celsius below standard conditions.

Recall that the temperature coefficient was 0.28. This means that for every single drift from the NOTC, our solar panel system will produce 0.28% greater voltage.

Firstly, we multiply the VOC of the panel with the temperature coefficient of the VOC.

The answer we get is 0.134624.

This answer is the change in voltage for every unit drop in temperature. Then, multiply this value by the temperature difference we obtained earlier. (77.3)

If you’ve been able to follow so far, you know that this is the voltage our panel will produce above its VOC voltage on the coldest day ever recorded. After that, add the two values to get the complete voltage the solar panel system will produce.

58.486 volts is the answer which you will get.
Then, multiply this by the maximum string number (12) we obtained earlier to get the maximum voltage, which we can get on a cold day.

That’s a very high voltage! As we can see, it is well above the maximum operating voltage of 600 volts, which means that a very cold day will potentially damage your inverter.
To combat this problem, we need to make adjustments to our maximum string size. Each panel has a true voltage of 58.486. Therefore, subtracting two panels will cause our value to fall within a range of 600 volts.
Thus, the maximum number of solar panels that you should have is 10. Any more and you risk damaging your inverter. The information presented above relates to temperatures that are extremely cold. But what about the other way round? What should you to do when the temperatures are quite high?

This is not a cause for worry. The NOCT falls within a range of 44-47 degrees Celsius during measurement, approximately 112-115 degrees Fahrenheit. Since most places don’t go beyond this range, no need for concern.

However, even if they did, your inverter isn’t at risk of damage, since warm temperatures only reduce the voltage. A decrease in voltage during high temperatures only brings your inverter further away from the danger zone.

High temperatures affect string size, though, so you should recheck to make sure that your string size is still within the optimal MPP range required for the highest efficiency. The calculations needed to determine that are the same ones we did before, just with different values.

1. In place of VOC, use the VMP value
2. Instead of the Temperature Coefficient of VOC, use the value of the Temperature Coefficient of Pmax
3. Don’t add the voltage compensation to the panel VMP to find out the true voltage; subtract it instead.
As an example, let’s assume a hypothetical highest temperature of 56.7 degrees Celsius. This is 12 degrees above our NOCT. Our VMP and Pmax Temperature Coefficient values are 39.28 volts and -0.377, respectively. Let’s do the math now:

1. (VMP)*(Temp Coefficient of PMax) = Voltage Change per unit Celsius

39.28*0.00377= 0.148

2. The total difference in Voltage due to the 12.7 degrees Celsius above the NOCT

0.148*12.7= 1.8796

3. This value subtracted from the VMP gives you the true panel voltage

39.28- 1.8796= 37.4004

4. Lastly, multiply this value with your minimum string size to obtain the total voltage that you can use as input.

37.4004*7 =261.8028

Recall that in step 1, we mentioned the lower end of the MPP as 270 volts. Since our obtained value of 261.8 falls below that, the ideal solution would be to increase the number of string panels to 8.

However, this calculation only works for the highest recorded temperature in our hypothetical area, a blistering 134 degrees Fahrenheit. Despite such a hot temperature, the only problem here is a system that operates slightly below optimal standards.
As you can see, the problem is insignificant and shouldn’t cause you any worry. Although 8 panels are the ideal string size, you can use 7 as well.

## Start String Sizing Your Solar Panel System

Congratulations! You’ve finally managed to reach the end of this exhaustive guide on stringing your panels.

Before we part, we’d like to ease your worries by telling you that solar panel systems are built while keeping these complex calculations in mind, so you don’t need to do any of the work yourself.

If you still need assistance in optimizing your solar energy system to match your needs, you can request for a free consultation with our experienced team.