What is String Sizing in PV Solar Design? Sean White Explains

String sizing is one of the most critical and foundational calculations in PV system design. At its core, string sizing is about understanding how solar modules behave under different temperature conditions and ensuring your system operates safely within the voltage limits of your inverter.

In this video, HeatSpring instructor Sean White breaks down the fundamentals of string sizing, walking through the practical side of these calculations – like using temperature coefficients from manufacturer spec sheets and applying them to real-world conditions.

Ready to dive deeper? Sean’s free “String Sizing Class” offers step-by-step video instruction on the math behind connecting PV modules in series. You’ll learn how to account for local temperature variations and calculate the maximum number of modules per string for any location. Sign up for Sean White’s free String Sizing course here!

Transcript below.

A lot of people want to know what string sizing is and what’s a string anyway? A string is connecting solar modules, otherwise known as solar panels together in series. When you connect things like that in series – batteries, solar modules – you increase the voltage by how many things that you have in series with each other.

One of the weird things about these magic crystals, otherwise known as solar cells, is that when it gets colder, the voltage goes up. So if you live in a really cold place, you’re going to have to do a different calculation to figure out how many solar modules you can put on a string. If you move from Hawaii all the way to Antarctica, like most of us have done at least once in our life, you have to string that system a little bit different. Or you can just leave those solar modules in Hawaii. They’ll appreciate it. It’s very nice over there.

So anyway, what you do for these calculations – there’s a couple of different ways of doing it – but the main way that people deal with it is there are these things called coefficients, such as the temperature coefficient for voltage that you’ll get from the solar manufacturer, and you figure out what difference in temperature that the solar module testing conditions were that the solar module was tested at, which is usually 25 degrees Celsius/77 degrees Fahrenheit.

Then if it gets 10 below, say for instance, 10 below is 35 degrees below 25 above. For some reason, people like to do this with the metric system. You’ll  multiply that 35 degree difference by this coefficient, and then you’ll figure out what percentage voltage increase you’ll get.

So we’ll go over that in my free string sizing class that costs you a couple of clicks of your mouse, so nothing’s really free. That electricity is probably about 0.0001 pennies. But look out pennies are going to be worth a lot coming up in the future. They’re getting rare.