Pros and Cons of 60-cell & 72-cell Solar Modules
The summary: 72-cell is about a foot taller, and they cost a bit less to mount in large-scale applications. However, both 60-cell and 72-cell solar modules use the same cell technology, and they work out to the same price from a cost-per-watt perspective. Either option can be used in residential or commercial installs – the ideal choice depends on your array layout and space constraints where you will mount your system.
But when it comes down to choosing your solar modules, the number of cells included in the panel isn’t the most important factor. Cost, aesthetics, warranty, efficiency, and durability should all be taken into account when selecting the right PV panels. In this article, we’ll discuss some differences between 60-cell solar panels and 72-cell solar panels as well as their pros and cons.
Background about solar modules
Solar modules are made from blocks of silicon ingots that are cut into wafers. Those are the squares that you see that make up the modules.
Each solar cell (monocrystalline) produces about 0.55 volt. Multiple cells are wired in series, plus to minus, to make a solar module. Wiring the cells in series increases the PV panel voltage to a usable level. The more cells that are wired in series, the higher the voltage.
12V and 24V nominal solar modules
For example, if 36 cells are wired in series, you get an output of about 19.8 volts. Remember that solar first got its start in the battery-based off-grid world, where PV panels were built to charge battery banks. The 36-cell solar panel that output 19.8V is perfect for charging a 12-volt battery bank. Since you need a higher voltage to charge a battery, a 36-cell solar panel is called a 12-volt nominal panel, it’s designed to charge a 12-volt battery.
Likewise, a solar panel with twice as many cells, 72 cells, outputs about 39.6 volts. And it’s great for charging a 24-volt battery bank. So it’s called a 24-volt nominal solar panel.
If you have a 48-volt battery bank, since there are few companies produce 48-volt solar panels, in that case, you need wire multiple PV panels together in series, either 4 12V nominal panels or 2 24V nominal panels, which will efficiently charge a 48V battery bank even in very hot weather when the voltage of PV panels drops low.
Grid-tied solar photovoltaic system
So all was well and good with figuring out what nominal voltage solar modules to use. Just make them match the nominal voltage of the batteries. But along came two different technologies that added an interesting twist to the next.
The first was grid-tied solar with a grid-tie inverter, you could now convert the DC voltage from the PV panels directly into AC to power your properties, no batteries required. So the restriction of 12V or 24V and 48V went away. This allows solar panel manufacturers to use however many cells they want to.
Today for solar modules up to above 320 watts, the industry settled on 60 cells. Using terminology from the battery world, that’s a 20V nominal panel. With an Open Circuit Voltage or Voc of around 41 volts. Grid-tied solar systems are able to string up to 20 pieces (1000/41/1.21=20) of 60-cell solar panels in series and stay within the electrical code restriction of staying under 1000VDC, even when considering the cold temperature. If they were using 24V 72-cell solar panels, they would be limited to only 16 pieces (1000/49.5/1.21=16) in series in cold environments.
PWM & MPPT Charge Controllers
The second change was still in the battery based world, with solar charge controllers that are used to manage to put the power from the PV panels into the batteries. Early on, the pulse width modulated (PWM) charge controllers had to match the nominal voltage between the photovoltaic panels and the battery bank. Maximum power point tracking (MPPT) solar charge controllers came onto the scene and allow you to take a higher voltage input and reduce the charge controller voltage output to correctly charge a battery bank while increasing the output current, eliminating any power loss due to forcing PV panels to operate at whatever voltage the battery bank was actually at.
This opened up the possibility to use any cell count solar modules, as long as the voltage was higher than the battery bank. Although some specialized charge controllers allow you to charge with a lower voltage solar panel, most of them require a higher voltage.
Since 60-cell “grid-tie” solar panels are so common now, their price per watt has dropped much lower than the standard 12V nominal panels, making them an attractive option to charge battery banks. Just remember, you have to use a more expensive MPPT charge controller to do it.
Also, note that you cannot use a single 60-cell solar panel to charge a 24V battery bank. You need at least 1 72-cell solar panel or 2 60-cell panels in series to have a voltage high enough to charge 24V. If you’re in a cold environment, most 150V MPPT solar charge controllers limit you up to only 2 72-cell panels in series, but you can do up to 3 6-cell panels in series and still stay within that 150-volt limitation.
Advantages of higher wattage solar modules
So, what about the 72-cell 24-volt solar modules for grid-tied systems? Well, technology and codes move forward, and newer revision of the electric codes now allow for 1500 volts strings for grid-tie commercial and utility-scale systems.
Also, grid-tie inverters are getting bigger and bigger, allowing for extremely high wattages is 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.
The cost of installing 60-cell vs. 72-cell panels may also vary a bit for your installation. 72-cell solar panels tend to be cheaper to install on a large scale, which is why they’re more common for commercial applications. Because each panel has more solar cells, you can typically install fewer panels to generate the same amount of electricity. Fewer panels mean less racking is necessary, which helps cut down on overall equipment and installation costs.
On the other hand, 60-cell panels are lighter and easier to install which is why they’re more common for residential rooftop installations. This can mean lower labor costs for the installation. For large, commercial installations, the extra size and weight of a 72-cell panel aren’t as much of an obstacle because cranes are often used to lift the equipment to the rooftop.
Exceptions
There’s one scenario where 72-cell solar panels are less common. This exception comes when you build a solar system around micro-inverters.
Micro-inverters work on a 1-to-1/2/4 system, where each inverter is hooked up to an individual PV panel(or 2 panels, sometimes 4 panels). The benefit of this type of system is that it is modular: if one part stops working, it doesn’t affect the rest of your array. This makes it easy to repair and expand your system.
Micro-inverters have a cap on how much AC wattage they can handle. If PV panels produce more power than the micro-inverter can process, the panels would still work, but you’re simply wasting electricity – and by extension, wasting money. You’d essentially pay a higher price for your panels without getting the benefit of additional output.
More factors to consider when choosing solar modules
When it comes down to choosing your solar modules, the number of cells included in the panel isn’t the most important factor. Cost, aesthetics, warranty, efficiency, and durability should all be taken into account when selecting the right PV panels for your home or business.
For the cost, it’s a good idea to compare your 60-cell and 72-cell panel options on a cost-per-watt ($/W) basis. Similar to looking at dollars per square foot when shopping for a home, looking at $/W shows you what the cost of a turnkey solar panel installation is on a per-watt basis. Looking at this factor will allow you to standardize the price of installations using either 60-cell and 72-cell panels, especially if you’re comparing different system sizes.
PV panels come in a variety of cell colors (dark blue, blue, black), cell shapes (pseudo square, full square), backsheet colors (white, black) and frame colors (silver, black). If aesthetics are a concern, make sure to look at an image of the panel before signing the contract, or ask your installer for photos of their past installations with the same equipment to get a feel for how they will look on your roof. If you’re looking for the sleekest appearance possible, there are many black-on-black panel options. You may pay more for these options, but it can be worth it.
You may also ask yourselves a few questions
- Are you building an off-grid or grid-tie solar system?
- If it is off the grid, what voltage battery bank are you trying to charge?
- Do you have a physical size restriction that would limit the size of the solar module?
- What equipment do you want to support the solar modules?
- Will a higher voltage module restrict the number of modules you can use in the solar system?
