Solar panel manufacturers are always competing against each other to squeeze the most power out of the same size solar panels.
If you installed solar panels on your home 10 years ago, you probably had panels with efficiency in the mid-to-high teens (by percentage of light converted to electricity).
Today, you can commonly find solar panels that convert more than 20% of the incoming light into electricity, with the current best residential panels hitting nearly 23% efficiency. With an increase from 16% efficiency to 22% efficiency, you would need 37% fewer solar panels to generate the same amount of power. That’s a lot less roof space, and lower installation costs.
Half-cut solar panels are one of the latest manufacturing innovations to improve solar panel efficiency. They are exactly what they sound like: a standard size solar cell cut in half. It might seem weird that such a seemingly simple thing would make a better solar panel, but there are two ways that half-cut cells improve panel efficiency.
This article will explain how half-cut cells work, but first let’s talk about conventional solar panels.
Most solar panels on the market are made with crystalline silicon cells that is monocrystalline or polycrystalline. Both types are produced differently and have different advantages - read this article to learn more - but with both types, the process starts with a solid block of silicon that is sliced thinly (like a ham) to create individual cells.
A solar panel is made of individual solar cells that are wired together. Each cell generates a little bit of voltage (about half a volt) and electrical current, but when lots of cells are wired together - such as in the kind of solar panel you would install on your roof - the combined output of those small cells might add up to as much as 60 volts and 400 watts.
Solar cells come in different sizes. A common size is 6 inches square. At that size, a residential solar panel will have 60 cells, and a larger panel designed for the utility or commercial market will have 72 cells.
Not all panels have 60 cells. For example, SunPower cuts their cells differently, and uses 96 small cells in their residential solar panels. There is also a trend toward larger solar cells, which has the potential to reduce manufacturing costs.
With half-cut solar cells, the initial manufacturing process is the same. That is, the solar cells are still manufactured to a conventional size, such as 6 inches square.
The next step is to saw the cells in half. They don’t use a chainsaw like in the photo above, but technology that’s a little more sophisticated - often a diamond wire saw or even a laser.
The resulting cells might might now be 6"x3" rather than 6"x6". This means that the finished solar panel would have 120 cells instead of 60.
The overall dimensions of a solar panel with half-cut cells is the same. The individual cells are visibly smaller (ie. half the size) and the spacing between cells is reduced as well.
The most noticable difference is a distinct line through the middle of the panel:
This line exists because the panel is electrically wired into two separate halves that combine their electrical current. The reason for this is described later in this article.
Earlier, we talked about the extra manufacturing step of sawing cells in half with a diamond wire or laser. Not only does the step add a little more complexity, but the additional cutting step has the potential to break cells and increase wastage. So why bother?
There’s two big reasons, and both work to increase overall panel efficiency.
You might remember from high school science class that electrical resistance increases when current increases. If you’ve ever use a hair dryer on full blast, you know that the wire can get pretty warm after a few minutes. That’s due to electrical resistance.
But if the hair dryer is on low power - that is, if you reduce the amount of current that the appliance is using - the wire doesn’t get quite as warm. This is because with less current draw, the electrical resistance drops too.
This is one reason why half-cut cells make more efficient panels. When a solar cell is half the size, it has half the surface area to collect light, and so it will generate only half the electricity of the original cell.
With the maximum electrical current cut in half, a half-cut cell has 25% less resistance than the original full-sized solar cell.
Twice as many cells are used to make a half-cut solar panel, which doubles the total current of the panel to make it equivalent to where it would be with full sized cells - except now slightly higher due to the improved efficiency.
Because the number of cells is doubled in a half-cut solar panel, if they were wired together in a conventional manner, the voltage would be doubled as well. (More high school science for you to remember!)
This would make the panel incompatible with standard inverters. To get around this, the panel is essentially divided into two halves that are wired together in parallel. This parallel wiring of the two halves allows the panel to have the same voltage of a conventional panel.
Because a half-cut solar panel is really two smaller panels working in concert, it has better shade tolerance than a conventional panel.
You can read our article on shading and solar panels to learn more, but a quirk of solar panels is that shade falling across a panel can drop the power output by more than the number of cells that are shaded.
That is, if a panel is half covered by shade, you might expect the power output to drop by half. In reality, it will drop by much more than that, because shaded solar cells have a tendency to block the electrical current of neighboring solar cells.
All panels have measures to help avoid this, but in certain cases a half-cut solar panel will have approximately double the shade resistance of a conventional panel.
The diagram below explains the wiring of a half-cut solar panel. As you can see, the panel is made of two halves that are wired together:
The improved shade tolerance of a half-cut solar panel depends on where the shadow falls. If it falls in such a way that is covers both halves, the benefit is reduced. However, if the shadow covers only one half, the other half doesn’t experience any impact at all, approximately doubling the shade tolerance of the panel.
Both these examples have disadvantages. For example, PERC cells that aren’t manufactured to a high standard may have higher failure rates, and heterojunction cells add manufacturing complexity.
Half-cut cells, in contrast, don’t rely on relatively exotic technologies. This may be one of the reasons why half-cut technology is currently being offered by so many manufacturers.
There are hundreds of solar panel manufacturers globally, and many of them offer panels with half-cut cells. Here’s a partial list of solar companies popular in the North America market that have half-cut solar panels in their product lineup:
This is not a complete list by any stretch, so if you are interested in a manufacturer not on this list, check to see if they offer half-cut cells.
Compared to some other technologies used to increase solar cell efficiency, half-cut cells don’t rely on exotic materials, but a change in manufacturing process. While this does add complexity and expense, the large number of manufacturers offering half-cut panels suggest that the cost-benefit may be worth it.
In addition, you can combine the half-cut approach with other technologies. For example, several manufacturers offer solar panels that use half-cut PERC cells, combining two different technologies to push panel efficiency higher.
Should you use half-cut solar panels for your project? The greater risk is higher manufacturing defects, but solar panel manufacturing across the industry is generally very good, and that’s especially true for the top tier solar manufacturers listed above. Because of that, the downside risk of half-cut cells is quite small.
In exchange, you get a solar panel that will potentially perform noticeably better in shaded conditions. If a shady roof is something you’re contending with, half-cut solar panels would be a good choice.