Solar Cell Efficiency Explained: How It’s Measured and Why It Matters

One of the most common questions in the solar industry is, “How efficient is it?”

While solar cell efficiency is standardized and relatively easy to measure, it can often be misleading in real-world applications.

What Does Solar Efficiency Mean?

Unlike appliances like furnaces or water heaters—where higher efficiency directly reduces energy costs—solar panel efficiency refers to the power the panel produces divided by its surface area (watts per square meter).

This means efficiency primarily determines the area required to generate a given amount of power, not necessarily how much energy it produces. Whether a panel is 5% efficient or 20% efficient, if it is rated for 10W, it will still deliver 10W.

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Types of Efficiency Measurements

Solar panel efficiency can be measured in different ways. The two primary types are:

Total Efficiency:

• Accounts for the panel’s outer dimensions, including spaces between cells, the frame, and junction boxes.

Aperture Efficiency:

• Focuses only on the panel's functional area, such as the active cell area, excluding non-generating components.

Aperture efficiency is always higher than total efficiency.

A Key Consideration When Choosing a Supplier

Be cautious when selecting a solar supplier. Some companies do not test finished products and only report individual cells' lab efficiency—not the final panel's actual performance. 

Additionally, some manufacturers use an "expected" or "average" output when stating efficiency or wattage, with very few guaranteeing a minimum panel power in their labels or marketing materials.

Why Efficiency Can Be Misleading

Efficiency Depends on Light Source

Efficiency ratings (for outdoor solar panels) only apply to outdoor conditions, as solar panels are tested using the full spectrum of sunlight. 

However, different light sources affect solar technologies in unique ways.

For example, an amorphous silicon thin-film solar panel with 6% efficiency outdoors can generate more power indoors under LED lighting than a monocrystalline silicon solar panel rated at 20% efficiency outdoors. 

This happens because:

• Amorphous silicon cells absorb light mainly in the visible spectrum, where LED lights emit the most energy.

• Monocrystalline silicon cells are more efficient outdoors because they also convert infrared light from the sun into power—but LED lights emit very little infrared light.

To learn more about the difference between solar in indoor and outdoor light, read our post, Outdoor vs Indoor Solar: The Key Differences.

Temperature Impacts Performance - Charging Efficiency

Efficiency alone isn’t enough when choosing a solar solution, especially for applications such as class 8 trucks powering electric APU batteries or trailers with all-electric refrigeration units. 

You must also consider the temperature coefficient, which indicates how much power output decreases as panel temperature rises. To learn more about how temperature affects solar panels check out our post, How Does Temperature Affect Solar Panels?

Understanding Standard Test Conditions (STC) in Solar Panel Ratings

Most solar panels are rated under standard test conditions (STC) at 25°C (77°F). 

The National Renewable Energy Laboratory (NREL) measures the performance of photovoltaic (PV) cells and modules using standard test conditions (STC) based on the IEC standard 60904-3. 

These conditions are: 

• Temperature: 25°C (77°F)
• Irradiance: 1000 W/m²
• Air mass: 1.5 (AM1.5)

However, in real-world applications, panel temperatures often reach 35–45°C (95–113°F) or more, even when the surrounding air is much cooler.

As a solar panel heats up, its efficiency decreases. The rate of this decrease depends on the solar cell technology used.

As shown above, a lower temperature coefficient means better performance in real-world conditions. 

A panel with a -0.27%/°C coefficient can produce 1-3% more power than a standard crystalline panel on a hot day, making it 1-3% more efficient under real world conditions (see table below).

Efficiency vs. Real-World Performance: What Matters More?

Consider this scenario: You are shopping for a 400W solar system for a truck’s electric APU batteries.

• Two panels—both with 20% efficiency—are rated at 400W.

• However, in real-world temperatures, the panel with a better temperature coefficient will provide more usable power, especially during hot times of the year when your APU system is working the hardest to keep you cool.

• Thus, while efficiency is important, temperature performance leads to increased energy yield, which matters more in practical applications.

Key Takeaways

• Solar efficiency measures power output per square meter—not how well a panel "saves energy." A panel with higher efficiency simply generates more power from a smaller surface area, but total wattage remains the same.

• Different solar technologies perform better under different light conditions. Monocrystalline silicon excels in direct sunlight, while amorphous silicon can outperform in low-light environments like indoor settings or cloudy conditions. Not sure which technology is the right choice for you? We walk through things to consider in, Amorphous vs. Crystalline: How To Determine The Best Solar Technology For Your Application, and can help you make the best choice for your needs.

• High efficiency doesn’t always mean better real-world performance. Factors like temperature coefficient are critical to the power a solar panel produces over time. 

• Temperature performance can be just as important as efficiency. A lower temperature coefficient means less power loss in high-heat conditions, making specific panels produce more power for trucking, off-grid power, and auxiliary battery charging applications.

• Choosing the right panel depends on your application. Some technologies, like amorphous silicon, are better for situations with variable light, while others, like high-efficiency crystalline panels, work best in stable, full-sun environments.

Read our post, Which Portable Solar Technology Is Best: A Primer, for a comparison of various solar technologies.

Final Thoughts: Efficiency is Just One Piece of the Puzzle

Efficiency is often the first metric people consider when evaluating solar panels, but it shouldn’t be the only one. 

While efficiency determines how much power a panel can generate relative to its size, real-world performance is influenced by temperature, light conditions, and application-specific needs.

At PowerFilm, we create solar solutions that fit your specific power needs. We don’t believe in a one-size-fits-all approach. The right solution delivers the most reliable, consistent power in your operating environment.

Find the Right Solar Solution for Your Needs

Are you looking to integrate solar into your fleet or commercial application? Contact us today. 

Our team will help you choose the right technology to maximize performance and ROI in real-world conditions.

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