In your research trying to understand solar power, you feel like solar panel efficiency is something you should be able to easily grasp. Efficiency should be easy to understand, right? Except the deeper you dig, the more confused you get. What’s the difference between solar cell efficiency and solar panel efficiency? Who can you trust when it comes to solar efficiency?
I got an email from someone purchasing solar panels from a supplier asking the difference between these two efficiencies. Helping people understand questions about solar is the whole point of this website, so after I answer a question via email – it often ends up here so that more people can learn.
Here’s the question:
My prospective supplier states that the Schuco 210 watt SMAU-1 panel has a cell efficiency of 16.8%, whereas your table gives a panel efficiency of 14.93%. What is the difference between panel and cell efficiency and which should be my guide?
Solar cell efficiency chart
Have you ever seen this image? If you search for “solar panel efficiency chart”, you’ll get this image (and my website). Can you even understand this chart?! It’s a whole bunch of colorful lines that look like they’re trending upward. That’s all I see and I specialize in solar efficiency. Let me explain this image and then help you understand the difference between cell and panel efficiencies.
The first thing you should notice is the title. This whole table shows solar research. Research results do not equal real life use. When I say real life, I mean for use by homeowners and maybe commercial building owners. I do not mean NASA. Sending satellites into space might justify putting some of these research results into practice. But we don’t live on Mars (yet). So this chart is good if you’re writing a thesis on cell efficiency gains over the last 35 years. But it’s not that helpful to the average homeowner.
Solar cells, panels, and arrays
The confusing image above is plotting cell efficiency – not panel efficiency. Solar cells are the little squares that you can sometimes see on solar panels. The panel is, well, the whole panel. (FYI: solar panel = module. They’re the same thing.)
You don’t need to know what a solar string is – unless you’re an installer. But essentially a typical home solar array consists of several panels connected in parallel as a string. The collection of strings makes up the array. Each solar panel is made up a collection of solar cells.
Cell efficiency is measured as the percentage output of energy to the theoretical amount of energy supplied by the Sun (hitting the cell). So if 100 Watts hits the cell (under Standard Testing Conditions, STC, which is laboratory conditions), and it spits out 16.8 Watts, then the cell efficiency is 16.8%. That’s about as simple as I can explain it (while still being technically correct).
So when a laboratory (like NREL) measures solar cell efficiency, that’s only measuring one of those little squares inside of a solar panel.
Compare panels on panel efficiency
My solar panel comparison table does not use cell efficient, but allows you to compare panel efficiency. This tells you how much energy one solar panel on a roof will produce compared to the space that panel takes up. It’s the PTC rating /squarefeet area.
Therefore, if a supplier (or installer) tries to sell you on the cell efficiency, ask him to give you the cell efficiency of his competitor’s panels. I bet you he can’t. So that cell efficiency number becomes meaningless without context or a point of comparison.
You can find that point of comparison in my solar comparison table which allows you to compare the panel efficiency of more than
1,700 8,700, 10,300, 13,700 17,500 solar panels. These measurements are standardized across all panels using the data that California Solar Initiative uses to calculate solar rebates.
After you find out how your solar panels stack up against the competition on efficiency, contact a solar installer and find out how much money you can save by having those panels installed on your roof.