Originally Published 4-26-2019
One of the most common questions we hear is whether solar panels can work through a window.
The short answer is yes, but performance drops compared to direct outdoor exposure. How much power a panel produces depends on the type of glass, coatings, tint, and the solar technology used.
In this blog post, we break down when solar behind a window works, when it does not, and which technologies perform best.
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Why Windows Reduce Solar Panel Performance
Solar panels generate power by converting sunlight into electrical energy. When that sunlight passes through a window, it changes before reaching the panel.
The chart below shows how different types of window glass filter sunlight across wavelengths. Clear glass allows most visible light to pass through but still blocks part of the solar spectrum. Low-E glass further reduces transmission, especially in the infrared range.
Windows do not just reduce the amount of light. They also change the type of light that reaches the panel.
This matters because many solar technologies rely heavily on infrared light to generate power. This concept is explored further in "Solar Intensity: Looks Can Be Deceiving", which compares visible brightness and usable solar energy.
Crystalline silicon technologies capture a large portion of energy from wavelengths between 800 and 1200 nm. Low-E coatings reduce transmission in that range, leading to lower power output behind modern windows.
Amorphous silicon behaves differently.
- It collects energy primarily from the visible spectrum (400–700 nm).
- These wavelengths pass through glass more easily.
- This results in less performance loss behind windows.
Light type is only part of the story. The angle of sunlight also plays a major role.
In addition to spectral losses, reflection also reduces performance as the angle of sunlight changes.
When sunlight hits a window directly, most of it passes through. As the angle increases, more light reflects off the surface instead of entering.
- Transmission stays fairly stable up to about 60 degrees.
- Beyond that, reflection increases quickly.
- This reduces the usable light reaching the panel.
Several factors contribute to reduced output:
- Spectral filtering: Glass blocks key wavelengths used by solar panels.
- Low-E coatings: Reflect infrared energy.
- Tinting and lamination: Further limit light transmission.
- Reflection losses: Light reflects away at steeper angles.
Do Solar Panels Work Through Windows?
Yes, solar panels can generate electricity through windows, but at a lower efficiency.
Solar panels behind windows receive less usable sunlight, which reduces output. For high-power applications, this can be a problem.
For a deeper look at how light intensity affects solar performance, see "Solar Intensity: Looks Can Be Deceiving".
You can also explore "Outdoor vs Indoor Solar: The Key Differences" to better understand how indoor environments affect solar output.
When Solar Panels Can Work Behind a Window
Low-Power or Maintenance Charging
Solar panels behind windows work best when steady, low-level energy matters more than peak output.
The chart below shows how an amorphous silicon panel performs behind a window compared to one mounted outside.
Figure 3: Solar output behind a window varies significantly by orientation, with north-facing installations relying almost entirely on low diffuse light. West-facing results are not shown due to building limitations, but performance is expected to be similar to east-facing results. Indoor/outdoor values represent the ratio of the current measured behind the window compared to the same panel in the same position outside.
Even in favorable conditions, output is much lower.
- South-facing windows perform best but still fall short of outdoor output.
- East-facing windows produce more power early, then drop quickly.
- North-facing windows rely on diffuse light and produce very little.
- The performance presented is the best case for a panel mounted flat against the inside of the glass. If there is a significant air gap between the panel and the glass or if the panel is at a different angle than the glass, reflection losses will increase.
Performance varies widely by orientation. South-facing windows can approach outdoor output under ideal conditions, while east and north-facing windows often drop to 7%-15% of outdoor output.
This makes it suitable for:
- Maintaining battery charge.
- Offsetting small loads.
- Powering low-energy electronics.
This is a strong fit for environments where sunlight is filtered, indirect, or inconsistent.
Vehicle Interiors
Solar panels are often placed behind windshields to maintain batteries or power small systems.
Figure 4: Solar panels placed behind vehicle glass can lose 30–40% of available light, with tinting causing even greater reductions.
The chart above compares a panel behind a vehicle window to one in direct sunlight.
- Behind a window placement reduces available light by 30% to 40%.
- Tinting further reduces performance.
- Even a light tint can significantly reduce output.
In extreme cases:
- A panel under a tinted sunroof may receive only 5% to 8% of available light.
Solar behind vehicle windows works best for:
- Battery maintenance
- Ventilation systems
- Low-power electronics
Indoor or Semi-Enclosed Spaces
Rooms with large windows and steady sunlight can support small solar applications.
These setups are not ideal for high-power systems, but they can work when energy needs are modest.
When Solar Panels Should Not Be Installed Behind a Window
Solar panels behind a window are not recommended when:
- Full power output is required.
- Windows have heavy tint or Low-E coatings.
- Sunlight is inconsistent.
- The application is critical.
In these cases, outdoor installation delivers much better performance.
Are Amorphous Silicon Panels Better Than Other Solar Technologies Behind Windows?
In most cases, yes.
Amorphous silicon panels perform better under filtered light.
- They work well in low-light environments.
- They handle diffuse light more effectively.
- They lose less performance behind glass.
This makes them a strong choice for solar applications behind windows, where other technologies struggle.
Key Takeaways
- Solar panels work behind windows, but the output is reduced.
- Glass type and coatings have a major impact.
- Performance drops further with angle and reflection.
- Solar panels behind a window work best for low-power applications.
- Amorphous silicon performs better than other technologies in these conditions.
Final Thoughts
If you plan to use solar behind a window, the details matter. If you’re comparing indoor and outdoor performance, "Outdoor vs Indoor Solar: The Key Differences" breaks down what to expect in each environment. Window type, lighting conditions, and panel technology all affect performance.
PowerFilm helps customers design solar solutions for real-world environments, including low-light and behind-window applications. Our team helps you select the right technology and avoid performance gaps early.
Contact our team to design a solar solution that performs reliably in real-world conditions.
FAQ: Solar Panels and Windows
Do solar panels work indoors near a window?
Yes, but output is limited. Solar panels near a window rely on indirect, filtered light, which reduces their performance. They work best for low-power or maintenance charging, not high-energy use.
Do solar panels work through regular windows?
Yes. Solar panels work through clear windows, but they produce less power because the glass filters and reflects part of the sunlight.
Does tinted or Low-E window glass affect solar performance?
Yes. Tinting and Low-E coatings significantly reduce the amount of usable solar energy reaching a panel, often lowering output more than standard clear glass does.
Will solar panels work behind a windshield?
They can, but performance is limited. Windshields are designed to block UV and infrared light, reducing the usable energy available for solar charging.
Is solar behind a window suitable for high-power applications?
No. Solar behind a window is best for low-power or maintenance charging. Applications that require full output should use panels outdoors in direct sunlight.
Should I install a solar panel behind a window or outside?
If possible, install the panel outside. Direct sunlight provides much higher output and more consistent performance. Behind-window placement only makes sense for low-power or space-limited applications.
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