A Bit of History
A solar panel is just a bunch of individual solar cells wired together in series. The more cells in a panel, the larger both its size and wattage will be.
Though it’s possible to make a solar panel composed of as many cells as you like, practically speaking, if you’re in the market for a solar system, you’ll be encountering just two options.
60-cell panels, which will fall between 260-330 watts
72-cell panels, which will fall between 360-400 watts
Though a lot of folks may naturally assume that bigger is better, when it comes to solar installations, the facts are a bit more complicated.
It’s the total wattage of your entire system that matters, not the output of each individual panel. And, while it’s true that you can build a system of any given wattage using fewer larger panels than smaller ones, there are a lot of reasons that 60-cell panels make more economic sense for residential and even most commercial solar projects.
Indeed, you may be surprised to learn that, while 72-cell panels have been around since the industry’s inception, 60-cell panels are a fairly recent market innovation.
And the change in the solar industry that suddenly created a demand for the smaller size—not coincidentally—was also instrumental in giving residential and smaller commercial solar projects the ability to provide electricity at a cheaper rate than utility companies.
The Storage Problem
The biggest problem solar energy has always faced is that the sun doesn’t rise and set in accordance with our energy needs. Homeowners use the most electricity in the evening, at which point the sun is either too low in the sky to produce any appreciable power or not shining at all.
Even after everyone’s sound asleep when there’s no sunlight at all, some energy is required to keep refrigerators and other electrical devices running.
The afternoon, when the sun shines at its brightest, on the other hand, is exactly when people tend to be most away from home and, hence, use the least electricity.
The upshot is that, in order for solar power to be more than just a costly novelty, there has to be a way to store the energy generated at peak-production hours for later use.
First Solution: Batteries
In the early days of the solar industry, batteries were the only solution. As a result, everything the industry did in those days revolved around them.
Now the key to understanding early panel sizes is that, for any given battery voltage, there will be an optimal higher voltage for the system you’re using to charge. If your charging system is below optimal voltage, you’ll lose efficiency, and if it's higher, you won’t gain any.
Each (monocrystalline) solar cell produces about 0.55 volts, so a 72-cell panel will produce 39.6 volts. And that turns out to be ideal for charging a 24-volt battery. Similarly, a 36-cell panel would produce 19.8 volts, which is perfect if you’ve got a 12-volt battery to charge it.
Since batteries can be either 12 or 24-volt, both 36 and 72-cell panels were in demand from the very beginning. Indeed, 36 and 72-cell panels were even called 12-volt optimal and 24-volt optimal, respectively—reflecting the essential link between panels and battery size.
The upshot is that, because solar-storage batteries didn’t come in any size between 12 and 24-volt, 60-cell panels made no commercial sense in the early days. On the one hand, they’d produced less voltage than you’d want for a 24-volt battery. But, on the other, if you had a 12-volt battery, they’d cost more than a 36-cell panel without providing any commensurate benefits.
But all of that changed when solar installations started connecting to the grid, so that any excess energy produced during the day could be harnessed by your local utility company.
Net Metering
Many states, including Pennsylvania, adopted net metering laws that require utility companies to reimburse homeowners at market price for any solar energy sent back into the grid.
Net metering, in effect, meant that surplus solar energy now could be stored for later use without the need for a battery and, hence, eliminated the battery-imposed limitations on panel size.
72-cell panels are still standard for large-scale installations. The larger size means that fewer panels need to be used. And, because large-scale projects use heavy machinery to get everything where it needs to go, there’s no significant increase in labor costs.
The 60-Cell Advantage
But the size of 72-cell panels turns out to create huge and costly headaches for residential installations. And likewise for the majority of commercial ones, which aren’t that much bigger.
60-cell panels are smaller and lighter, making them easier to install and resulting in a marked decrease in labor costs.
60-cell panels are typically around 66” x 40” and weigh around 40 pounds. Whereas 72-cell panels tend to be around the same width but an extra foot in length. They weigh in at around 48 pounds.
These differences may not sound like much. But if you’re employing a crew to carry and position panels, they become enormous.
A 72-cell panel will likely be taller than your biggest crew member So, lugging them up to a roof which may be two or more stories up is going to be labor intensive and, hence, will significantly increase installation costs.
60-cell panels provide greater design flexibility.
One great challenge in designing a solar installation is often maximizing the amount of power produced so that it meets all of the homeowner’s energy needs.
Roof space is always limited, so the design flexibility provided by smaller 60-cell panels can be a huge advantage. Even more so given that many states and localities have fire access codes that put major additional constraints on panel placement.
Using 60 as opposed to 72-cell panels frequently allows for the installation of one or more extra rows, translating into a larger system able to meet all the homeowner’s energy needs.
Transport
To be honest, trucking companies aren’t even terribly fond of the non-standard sized pallets required for the 60-cell panels which are the solar industry norm. But 72-cell panels create even bigger shipping headaches
A 78” long pallet is too big to be turned inside a freight truck. And anyone who’s ever worked a forklift will tell you that balancing a 78” pallet along its length is going to be inordinately challenging.
60-Cell Panels Make Sense
And there you have it.
Once panel size was no longer limited by the need to charge a battery, other sizes beside 36 and 72 cells became commercially feasible.
The size of each individual panel is irrelevant to meeting your energy needs. All that matters is the total size of your overall system.
As a result, because they lower labor and transportation costs as well as offering better design flexibility, 60-cell panels quickly became the industry standard for residential and all but the largest commercial solar installations.