One aspect of designing a solar PV system that is often confusing, is calculating how many solar panels you can connect in series per string. This is referred to as string size. Contact online >>
One aspect of designing a solar PV system that is often confusing, is calculating how many solar panels you can connect in series per string. This is referred to as string size.
If you are unfamiliar with the terms "series" and "string", it could be a good idea to head over to our article Introduction to Electricity for Solar PV Systems to get familiar with the electrical terminology used in solar.
This article will focus on calculating string size when using string inverters or charge controllers. If you are planning to use DC optimizers or Micro-inverters in your system then this information does not apply. Optimizers and micro-inverters have specific rules around how many panels can be connected to them, and how they can be connected together. The rules vary between manufacturers and components, and can be found in the manufacturer design guidelines and product datasheets.
There are two main steps in calculating string size.
If the maximum input voltage of your inverter is exceeded on a cold day, the inverter can be damaged. Even if the inverter is not damaged by over voltage, having too many panels in a string may void the inverter warranty, so that you are not covered for other inverter issues.
To make sure you don''t exceed the maximum voltage of your inverter, the first thing you need to understand is how the voltage of the solar panels changes with temperature.
The voltage of a solar panel is not fixed. As the temperature of a panel increases, its voltage decreases, and as its temperature decreases, its voltage increases.
The rate at which the open circuit voltage of a solar panel will change as its temperature changes is defined by the Temperature Coefficient of Voc. You can always find this value on the solar panel datasheet.
The temperature coefficient will be given in %/°C, (percentage per degree celsius). That is, is the percentage that Voc will rise, for every degree celsius the temperature of the panel drops.
For example, if you have a solar panel that has a Voc (at STC) of 40V, and a Temperature Coefficient of 0.27%/°C. Then for every degree celsius drop in panel cell temperature, the voltage will rise by:
Since STC is at 25°C, then at 24°C, the new Voc would be 40.108V.
Some datasheets will give the temperature coefficient in mV/°C. In this case you can convert to %/°C by dividing the mV/°C figure by the Voc value. Just be sure to convert from mV to V first.
Since you need to find your maximum string voltage, and you know that voltage increases as temperature decreases, you need to find out what your lowest expected temperature is.
Luckily there are standards that tell you how to calculate this number, and websites that will tell you which temperature to use at your location.
Firstly, the temperature you need to use is the mean of annual extreme low temperatures, or to put it another way, it is the average of all the coldest temperatures from previous years. This is the figure recommended both by the NEC.
To find this temperature at your location, you can use one of the following:
If for some reason you cannot find the mean low temperature for your location, an alternative is to use the lowest historical temperature instead. This is a little more conservative, but will always be within the standards and warranty requirements.
So now you know the solar panel Voc and Temperature coefficient, and the lowest expected temperature for your location. You can now calculate the voltage of a panel at that temperature, which is the maximum voltage of one panel.
Assume you had the following values:
First, find the difference between STC temperature (25°) and your expected low temperature
Multiply this by the temperature coefficient. Ignore the – sign, since you are only looking for the change in Voc here
Now increase Voc(STC) by this percentage
(You may see calculations elsewhere that look slightly different to this method, don''t be concerned as these are just different ways of showing the same thing, and (as long as they haven''t made an error!) should give the same results. The method here is an attempt to break this calculation down as simply as possible.)
Once you have the max Voc of one panel, all you have to do is divide your inverter maximum voltage by this value, and then round down to the nearest whole number.
For example, using the example above with a 600V inverter:
So this means if you connected 13.41 panels to your inverter you would be right at the inverter’s voltage limit. Now obviously you can''t have 0.41 of a panel, so you always round down to the nearest whole number. In this case, 13 panels per string is the maximum.
Now that you know what the maximum string size you can have is, you also need to calculate the minimum string size. Safety and inverter warranty are not a concern here like with maximum string size, but your inverter has a minimum input voltage which it can run at, and you want to make sure your inverter will continue to run on the hottest days of the year, or else you will be losing valuable generation.
The method is very similar to calculating maximum string size. The main differences are that Vmp is used instead of Voc, we have to choose a maximum temperature instead of a minimum temperature, and in the final step we have to round up instead of down.
When selecting a maximum temperature to use in the calculations, you can again use SolarABCs. You will see two options for High Temp, 0.4% and 2%. Select the 2% figure.
As with the minimum temperature, if you are unable to find the above values for your area, you can always use the highest recorded temperature instead. This will just give a more conservative (longer) minimum string size.
You now have the maximum ambient temperature for your location, but you also need to consider that solar panels operate at temperatures much higher than ambient. How much hotter they get depends on the mounting method, since this affects the ventilation of the panels. The following rules of thumb can be used.
So if we assume a roof-mounted system near Orlando International Airport, then from the image above our cell temperature is:
Assume the following:Vmp: 34.7VPower Temperature coefficient: -0.34%/°C
First, find the difference between STC temperature (25°) and your expected high temperature
Multiply this by the temperature coefficient. Use the temperature coefficient for Vmp if it is on the datasheet, if not use the power temperature coefficient.
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