Split cell panels, half cut panels, 120-cell panels. What are they, and do you want them?
Traditional solar panels have 60 cells – 6 wide and 10 tall. Larger, commercial, panels would have 72 – 6 wide by 12 tall – which is why the commercial panels are generally 20% taller, 20% heavier, and produce 20% more power.
Split cell/half-cut technology is pretty simple – cut each cell in half. So, instead of having 60 square cells, you have 120 rectangular ones. The panel stays almost identical in terms of size and weight.
REC was the first company to release split cell/half-cut panels, in 2014, but now it’s standard with most panels.
What’s the difference?
Before I get into the numbers, I’d recommend reading my Electricity Basics article to understand voltage, current, parallel, and series.
Each cell in a solar panel produces 0.5V and, let’s say, 10A as an example. These are approximate figures, and vary depending on the panel and the weather conditions, but it makes the maths easy and it’s not too far from reality. When you cut a cell in half, the voltage stays at 0.5V, but the current halves to 5A.
A 300W 60-cell panel is made by having 60 cells in series, in three sub-strings. This means the total voltage for the panel is 0.5V x 60 = 30V, and the total current is 10A, for a power rating of 30 x 10 = 300W.
A 300W 120-cell panel is made by having 60 cells in series, and having that in parallel with another 60 cells in series (each with three sub-strings, for a total of six). This effectively turns the panel into two half-sized panels wired in parallel. Parallel strings have the same voltage, but current is additive. So each half panel produces 60 x 0.5V = 30V and 5A, so in parallel they produce 30V and 5A + 5A = 10A – again, for a total of 300W.
What are the pros and cons?
Shading is the main advantage. If a cell is completely shaded, the current drops to ~0. As the current through a whole string cannot go higher than the current through any one cell (think of a narrow/blocked water pipe), having one shaded cell with 0 current means the whole string has 0A. Not ideal.
Using rough numbers, in a 300W 60-cell panel, completely shading one cell means you lose the full power from one sub-string, meaning you’d lose 1/3 of the total power of the panel (as there are three sub-strings). Likewise, in a 120-cell panel, if one cell is completely shaded, you’d lose the full panel from one sub-string. However, as there are six sub-strings in split cell panels, you only lose 1/6 of the total power. So, a 300W 60-cell panel, with one cell fully shaded, would produce 200W. A 300W 120-cell panel, with the exact same shading, would produce 250W.
A more extreme, but highly possible, example: imagine you had 20 panels, in portrait, in a row (20 x 300 = 6kW), and the bottom cell of all the panels were shaded (e.g. by a wall). If they were 60-cell panels, you could potentially lose all the power (0kW) – as all three sub-strings have at least two panels shaded, no power would flow through any of the sub-strings. If they were 120-cell panels, the top half of the panels (the top three sub-strings) would still work fine, meaning you’d still get 3kW.
There are other advantages too. Shaded cells often overheat (see my article on hot spots), and this heat can cause cell failure. A shaded cell in a 120-cell panel will have fewer cells connected to it, meaning less energy being dissipated through it, meaning less damage. Of course, it does have less area to disperse the heat, but overall it is an improvement.
So, what’s the downside? Well, there are more joins, more solders, more cells – arguably more ways the panel can break! This is one of many reasons why it’s important to get a good quality panel with high levels of quality assurance, such as Q CELLS, Phono, or Canadian.