The Benchmark of PV Testing: What Exactly is AM1.5G?

At every stage of the photovoltaic industry, from R&D breakthroughs in the lab to quality control on the production line, and finally to the power generation assessment of operational power plants, one core reference standard is indispensable. As you delve deeper into PV technology, a specialized term—"AM1.5G"—repeatedly appears. It may seem like a mysterious code, but it is, in fact, the "common language" for the entire industry and the "benchmark standard" for measuring the performance of photovoltaic devices. So, what exactly does AM1.5G mean? And why does it hold such a pivotal position in photovoltaic testing?

I. Deconstructing AM1.5G: Starting from Sunlight's Journey Through the Atmosphere

To understand AM1.5G, we need to break it down into two parts: "AM" and "1.5G".

1. "AM" - Air Mass
   "AM" stands for "Air Mass". It does not refer to air pollution levels but is an astronomical and physical concept defined as the ratio of the path length sunlight takes through the atmosphere to reach the Earth's surface to the path length it would take if the sun were directly overhead (zenith).

   We can use a simple model to understand this: Imagine the Earth's surface at standard atmospheric pressure. When the sun is directly overhead (at a zenith angle of 0°), its rays come straight down, traveling the shortest possible path through the atmosphere. We define this state as AM1. Here, the "thickness of the atmosphere" the sunlight must penetrate is 1 unit.

   However, in reality, the sun is not always directly overhead. From sunrise to sunset, sunlight hits the ground at an angle. This oblique path means the light must travel through a thicker layer of the atmosphere to reach the surface. The longer this path, the more the sunlight is absorbed, scattered, and weakened by the atmosphere.

   

2. "1.5" - A Specific Path Length
   "1.5" is the specific value for this path length. AM1.5 means the actual path length of sunlight through the atmosphere is 1.5 times that of the vertical AM1 path. This corresponds to a sun angle of approximately 48.2° from the zenith.

   So why specifically 1.5, and not 1.2 or 2.0? This is a standardized average condition established by the scientific community based on global general conditions. AM1.5 is considered to reasonably represent the average annual solar illumination for mid-latitude regions (such as much of the USA, Europe, and China), making it a typical and universal model.

3. "G" - Global Irradiance
   The "G" stands for "Global," representing "Global Irradiance." This total irradiance includes two components:

   • Direct Radiation: Light coming straight from the direction of the sun.

   • Diffuse Radiation: Sunlight that has been scattered by air molecules, clouds, dust, etc., in the atmosphere and reaches the ground from all parts of the sky.

   Therefore, "AM1.5G" completely describes a standardized solar spectral condition: the total solar irradiance (including both direct and diffuse components) received at the Earth's surface when sunlight passes through 1.5 times the standard atmospheric thickness.

II. The Core Value of AM1.5G: Why is it the "Gold Standard" for PV Testing?

Having understood the definition of AM1.5G, let's explore its indispensable core value.

1. A Unified Benchmark for Performance Evaluation
   Imagine the chaos in the PV industry without a unified testing standard. One company tests its module's power at high noon under clear skies, while another tests under cloudy conditions or in the evening. Because the light conditions (spectral distribution and irradiance) are completely different, the resulting data would be incomparable. It would be like using an elastic ruler to measure length—the results would be unreliable.

   The introduction of AM1.5G was precisely to establish this "rigid standard." All laboratories, research institutions, and manufacturers worldwide must test key parameters of solar cells and modules—such as conversion efficiency, maximum power, and fill factor—under standard conditions simulating the AM1.5G spectrum and an irradiance of 1000 W/m². Only then can the performance of products from different technology routes and different manufacturers be compared on a fair basis. When a customer sees a module rated at 550W, they can be confident this number was measured under the globally recognized AM1.5G standard.

2. Defining Standard Test Conditions (STC)
   On PV module nameplates and datasheets, you will invariably find "Standard Test Conditions" (STC). This condition is precisely defined by three factors: the AM1.5G spectrum, an irradiance of 1000 W/m², and a cell temperature of 25°C. Among these, AM1.5G defines the quality of the "light"—i.e., the distribution of energy across different wavelengths in the solar spectrum—while 1000 W/m² defines the quantity of the "light," representing a near-maximum solar irradiance level received at ground level.

3. Guiding Technology R&D and Innovation
   For PV researchers and developers, the AM1.5G spectrum is the fundamental basis for designing and optimizing cell structures. Different materials (like crystalline silicon, thin-film, perovskite) respond to different wavelengths of light with varying efficiency. By deeply studying the energy distribution of the AM1.5G spectrum, engineers can strategically design anti-reflection layers, passivation layers, and absorption layers to help the cell capture and convert more photons under this "standard spectrum," thereby continuously pushing the ceiling of conversion efficiency.

III. The Specific Content of the AM1.5G Spectrum

A complete standard AM1.5G spectrum has its energy distributed across the ultraviolet, visible, and infrared regions. It precisely specifies the solar irradiance that should be present at each wavelength from 280 nanometers to 4000 nanometers. The total integrated area under this curve (the total irradiance) is standardized to 1000 Watts per square meter.

This spectrum accounts for various atmospheric attenuation effects, such as:

• Ozone absorption of ultraviolet light.

• Water vapor absorption of specific infrared bands.

• Rayleigh scattering (which causes the blue sky) affecting blue light.

• Scattering and absorption by aerosols and other particulates.

It is these complex physical processes that collectively shape the contour of the solar spectrum reaching the ground, and AM1.5G is the scientific summary and standardization of all this.

Conclusion

In summary, AM1.5G is by no means an obscure and difficult academic concept. Rather, it is the lifeline running through the technology, markets, and standard systems of the photovoltaic industry. It is the "artificial sun" in the laboratory, the "impartial referee" on the production line, and the technical link connecting PV professionals globally.

For marketing professionals in the PV device industry, a profound understanding of AM1.5G not only helps us clearly explain the source and reliability of product performance data to customers but also highlights our company's commitment to industry standards, technical rigor, and product quality. On the new journey towards the era of grid parity for PV, this precise "standard" will continue to guide the direction of technological innovation and industrial development.