Solar panels are made from pieces of silicon that are cut into square cells. Those are the squares that that make up solar panels. Each solar cell produces half a volt. Multiple cells are then wired in series to make a solar panel. Wiring the cells in series increases the solar panel voltage to a usable level. The more cells that are wired in series, the higher the voltage.
If 36 cells are wired in series, you get an output of about 18 volts. A 36 cell solar panel that outputs 18V is perfect for charging a 12V battery bank. Doesn’t sound intuitive? You need a higher voltage to charge a battery. For that reason, a 36 cell solar panel is called a 12V “nominal” panel, because it is designed to charge a 12V battery. Similarly, a solar panel with twice as many cells, 72 cells, outputs about 36 volts, and it is great for charging a 24V battery bank. So it is called a 24V nominal solar panel.
This distinction may or may not be important, depending on what kind of system you are trying to design.
With a grid-tie inverter, DC voltage from the solar panels converts directly into AC to power your house with no batteries required. So the restriction of 12V, 24V, and 48V disappears. This allowed the solar panel manufacturers to use however many cells they wanted to. For solar panels up to about 300W, the industry settled on 60 cells. Using the terminology from the battery world, that’s a 20V nominal panel.
With an Open Circuit Voltage, or Voc, of around 38V, grid-tie solar systems were able to string up to twelve or thirteen 60 cell solar panels in series and stay within the Electrical Code restriction of staying under 600V DC, even when taking temperature into consideration. If they were using 24V 72 cell panels, they would be limited to only 11 in series in cold environments, limiting their system size.
If you are using a battery system, modern technology has produced MPPT charge controllers. MPPT solar charge controllers (Maximum Power Point Tracking) came onto the scene and allowed you to take a higher voltage solar input and reduced the charge controller voltage output to correctly charge a battery bank, while increasing the output current – eliminating any power loss due to forcing solar panels to operate at whatever voltage the battery bank was actually at. This opened up the ability to use any cell count solar panels, as long as the voltage was higher than the battery bank.
72 cell grid-tie solar panels
So, what about the 72 cell 24V solar panels for grid-tie solar systems? Well, technology and codes move forward, and newer revisions of the electric codes now allow for 1000V strings for grid-tie commercial and utility scale systems.
Also, grid-tie inverters are getting bigger and bigger, allowing for extremely high wattages getting inverted. Since watts equals volts times amps, a way to increase the wattage of a solar panel is to increase the voltage by adding additional cells. Adding those 12 additional cells generally adds a foot of length to the solar panel. With larger commercial and utility scale projects, larger panel sizes are able to be used. So, 72 cell panels give them a way to regularly make solar panels over 300W. Higher wattage solar panels reduce the number of panels that are needed in the system to reach your target wattage, which can decrease the labor needed to install them.