The Whys of String Sizing

Let’s go over the whys of string sizing. What is it and why do we do it?

String Sizing Basics

String sizing is critical when you’re putting modules in series. Remember that when you put modules in series, the voltage goes up. It’s additive. 

Another thing that makes the voltage go up is cold temperatures. That means in places where it gets colder, you’re going to have higher voltage with the same number of modules.

When sizing strings in a solar design, you want to double check to see how much voltage each module will get. Then you can figure out how many modules we can put in series. 

On the other hand, when there’s hot temperatures, the voltage is lower. We don’t want to have too few modules in series, not because there’s some high voltage danger or shock hazard, but because we want our system to work efficiently. If there’s not enough voltage, our system won’t work efficiently and it might even turn off. 

Another benefit of having long strings is less combining. If you just had strings of two and you had 20 modules, that would be 10 strings. That would be 10 things to combine. 

Sometimes in the older days of solar, we would have strings of one and we would just combine a whole bunch of modules to do a 12-volt battery system. Nowadays, we’re seeing 20 modules in series with 1000V systems, depending on your temperature and your module. On utility-scale projects, we can see 30 in series for 1500V systems.

Let’s take a look at some of the different things.

An inverter has a working voltage window. As system designers, we want to stay inside of that voltage window. The problem with a system with voltage that’s too high is that you can cause a spark which would also void the warranty. We don’t want to void that warranty because warranties are valuable in case something goes wrong with your inverter that has nothing to do with going over voltage. Also, it’s not allowed by the National Electrical Code. 

Remember that when your voltage gets too high, that can be dangerous. If your voltage is too low, your PV system will likely be inefficient. You’ll have a higher voltage drop percentage, which means more power loss. Your system might turn off on a hot day, because high temperatures will lower the voltage of a PV system, making it not work at the maximum power point tracking voltage.

Operating at the maximum power point tracking voltage means there’s going to be a voltage where your inverter works most efficiently. You want to be able to operate at that most efficient part of your inverter. 

However, if you have a range of modules in series that you can design 8-10, I would recommend going for the higher number and putting 10 in series if you could.

It also depends on how many modules you can fit on your roof facing in a particular direction. Ultimately, the strings need to be sized just right. I have termed it – the Goldilocks Zone for PV string sizing – where we get it just right.

Then, the voltage is not too high, not too low –  it is just right! Just like with the three bears. It’s not too hot or too cold. The same thing goes with PV string sizing. If it’s too hot, the voltage is too low. If it’s too cold, the voltage is too high. We want it just right in that string sizing Goldilocks Zone.

String Sizing and Module-Level Power Electronics

With the relatively new rapid shutdown (NEC 690.12) rules in the NEC for PV systems on buildings, there are many systems where a string is really just one module going to a microinverter or an optimizer (dc-to-dc converter). In fact, SolarEdge and Enphase are the most popular products in North America for PV systems on buildings and they do not require string sizing, unless you consider matching 1 module to 1 electronic device. On the other hand, there are some module level electronic devices like Tigo, where they still pass the PV voltage through and you still have to do string sizing. There are new devices coming out all of the time, so make sure to read the instructions or ask the manufacturer if you need to do string sizing when you have module-level power electronics (MLPE). 

Where do we get the weather for electrical design? 

In the earlier days of the PV industry, people and inverter manufacturers were recommending the all-time record temperatures. There are reasons why we don’t have to be so severe these days. 

If it gets super cold, it’s probably dark. If it’s dark, the sun’s not out anyway. Therefore, you’re not going to worry about going over voltage. If the sun is out, it’s likely that the sun is heating up the module above ambient temperature. When the sun hits a PV module, its temperature might go up approximately 30 degrees Celsius which heats that module up quite a bit. 

In fact, I’ve seen modules go from zero degrees Celsius all the way up to 40 degrees Celsius on a cold day, just because the sun was hitting that module in Canada and it was heating it up. 

We all heard the stories about cooking an egg on a black surface, like the pavement, in the sunlight, just because it’s so hot. It’s like that with PV. When the sun hits it, it heats it up. That’s the reason why we don’t have to use that all-time record temperature.

One last thing I’d like to mention is when a PV inverter is turned on, it’s working at maximum power voltage or Vmp. So just in case, that’s going to be our insurance. 

Let’s say that the wind is blowing hard. It’s super sunny and that wind is blowing all the heat away from the modules. If the inverter’s turned on, it’s going to be a significant amount away from working at open circuit voltage or Voc. The PV array would be operating at maximum power voltage. At maximum power voltage, there’s likely not going to be a problem with string sizing when it gets a bit colder than the record low.

Let’s look at the hot temperature. What’s the problem with the hot temperature? That would be your shortest string. Your shortest string just means that we don’t want the inverter to turn off on a hot day or work at an inefficient point on a hot day.

When it gets hot, what temperature are we going to use there? One of the things about the hot temperatures is that it’s not going to void any warranties. It’s not going to be against code, like if a string is too long. It’s just not going to work efficiently. In fact, you can put a PV system inside of a building and hook it up perfectly code compliant and have it never work with one module in series on that inverter. It would just never turn on. It wouldn’t be against code. It wouldn’t be dangerous. It would just not be smart to have a PV system that doesn’t turn on or doesn’t turn on often. That’s what we’re talking about – having our PV system turn off when it gets hot. 

In fact, a lot of the inverter manufacturers will tell people who call up in the middle of the summer and complain that their PV system is turned off to go spray a hose on it. After they do the hose test, people call back to the inverter company and they say that the PV array just must have been dirty. The inverter companies say, “Nope, you cooled it off with the water. That’s why it worked.” There were too few modules in series. Make sure to have the right amount of modules in series, and you won’t have this problem. 

What temperatures should you use? 

The industry standard these days is to use the ASHRAE numbers. There’s actually an informational note now in the National Electrical Code that talks about the ASHRAE numbers. Remember that an informational note is sort of like a yield sign, one of those yellow traffic signs. It’s not required. It’s just a good idea. ASHRAE stands for the American Society of Heating Refrigeration and Air Conditioning Engineers. 

The solar industry has made it easy to access this information. Go to the “Expedited Permit Process” document through the website SolarABCs.org to get the solar reference points. 

Learn about string sizing and more in Sean’s Solar PV Boot Camp + NABCEP PV Associate Exam Prep course!