Solar Efficiency Basics

by Shawn | 5 Comments

Solar Efficiency BasicsTiersCompareMost EfficientTable

Solar Module imageSolar panel efficiency is simple, but there a few things you need to understand.

There are 3 main types of solar efficiency.

1.  Module Efficiency
2.  Area Efficiency (Density)
3.  Cell Efficiency

1.  Module efficiency measures how well a solar module (aka panel) converts the Sun’s energy into usable energy. If the Sun dumps 100 Watts of energy onto the module and the module spits out 15 Watts, the the module is said to have 15% module efficiency (15 W / 100 W = 0.15 = 15%). If you want to brag to your friends that you have a top tier efficiency panel, care about this.

2.  Area efficiency (aka Density) measures how much usable energy a module produces in a given area. It’s Watts per square foot, so the more Watts, the more energy you’ll get from a specific area (or available space on your roof). So if a module spits out 210 Watts in 15 square feet, it’s density is… c’mon, it’s Watts per square feet (and I made it an easy number). Okay, it’s 14 W/sqft (= 210 W / 15 sqft). If you have limited roof space, get the panel with the highest density.

Solar Cell image3. Cell efficiency is measured the same way as module efficiency, but only with a single cell. This is the number news media and blogs love to tout in their headlines as ‘record breaking’ and ‘highest efficiency achieved’. This number is generally not useful for the average consumer. If you work in a solar cell lab, don’t give up on increasing this number.

So that’s the basics of solar efficiency. Now, let’s make it even simpler by dividing solar panels into 5 Efficiency Tiers (or levels).

  • James F. Smith

    This seems a good place for a shout-out to Shawn at SRoeCo Solar, who very generously replied to my emailed plea for help in deciding whether to go with a Sungevity lease for our home, which has conditions that make us somewhat marginal. Shawn provided a thorough, measured response to my explanation of our circumstances — providing me with valuable disinterested advice. I’m still weighing a couple of factors including shade and roof condition, but Shawn helped me feel less gun-shy about plunging into a big investment because he offered some sensible benchmarks. It’s very hard to find independent advice to make these complicated choices.
    Thanks to you, Shawn!

  • Nick Spook


    I was wondering how it is possible that one solar module might have a higher efficiency than another while having a lower density. After all, the density of a module (power output per unit of surface area) is just the efficiency of that module multiplied by the solar irradiance (supplied power per unit area). So, there should be a simple linear relation between the two. It follows that if one module has a higher efficiency than another, then it automatically has a higher density as well.

    Kind regards,


    • Fair question Nick. The difference lies in the fact that two different measurements of output are used. ‘Efficiency’ is calculated using the laboratory measurement called STC (Standard Testing Conditions). Whereas, ‘density’ uses the measurement system based on more realistic outdoor conditions, PTC (PVUSA Testing Conditions). It’s this difference in output under lab conditions (STC) versus real-world conditions (PTC) that causes the non-linear relationship between ‘efficiency’ and ‘density’.

      You can read a little more about the difference between STC and PTC in my comment here.

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