Air Mass in Solar Energy: Definition, Types & Importance for PV Systems
Have you ever wondered why solar panels produce the most electricity at noon (in summer) and less in the morning or evening and winter, even when there’s no cloud? The answer lies in a simple but powerful concept called Air Mass (AM).
Air Mass defines how far sunlight travels through the Earth’s atmosphere before it reaches your solar panels. This journey through the air filters and scatters sunlight, changing both the intensity and the spectrum of solar radiation. In solar energy, understanding Air Mass is essential because it directly influences how much electricity panels can generate.
What is an Air Mass? (Definition)
Air Mass (AM) is the ratio of the actual path length (PO) sunlight takes through the atmosphere to the shortest possible path (ZO) (when the Sun is directly overhead).
Air Mass (AM) = PO/ZO
Or mathematically, it’s expressed as:
AM=1/cosθz
Where θz is the solar zenith angle (the angle between the Sun and a line perpendicular to the ground).
- AM = 1 → Sun directly overhead, shortest path.
- AM > 1 → Sun lower in the sky, longer atmospheric path.
The longer the path, the more the atmosphere absorbs and scatters sunlight, reducing the energy reaching your solar panels.
Types of Air Mass Values
The Air Mass value varies due to the relative change in position of the Earth and the Sun. Let us analyse some of the AM values:
AM0 — Solar Spectrum in Space
- Represents sunlight outside Earth’s atmosphere.
- Used for space solar arrays (satellites, ISS).
- Contains the maximum possible solar energy (~1,366 W/m², known as the solar constant).
AM1 — Sun Directly Overhead at Sea Level
- Theoretical condition (Sun at zenith).
- Very rare except near the equator around noon.
AM1.5 — The Global Standard
- Sun at ~48.2° from zenith.
- Standard testing spectrum for PV modules worldwide (IEC 60904, ASTM G173).
- Called AM1.5G (Global) because it includes both direct + diffuse radiation.
- Efficiency ratings you see on solar panels (e.g., 21%) are measured under AM1.5G conditions.
AM2, AM3, etc. — Lower Sun Angles
- Represent the Sun closer to the horizon (morning, evening, high latitudes).
- Longer air path → less irradiance, more red-shifted light.
| Air Mass | Description | Sun’s Position | Approx. Solar Altitude Angle |
|---|---|---|---|
| AM0 | Outside the Earth’s atmosphere (extraterrestrial solar radiation) | Used for satellite and space applications | — |
| AM1 | Sun directly overhead (shortest atmospheric path) | At the equator, noon | 90° |
| AM1.5 | Sun at ~48.2° from zenith | Standard reference for testing solar panels | ~41.8° above horizon |
| AM2 | The sun is lower in the sky | Morning/evening sunlight | ~30° above horizon |
| AM5–AM10 | Very long atmospheric path | Sunrise or sunset | <15° above horizon |
Why AM1.5 is the PV Testing Standard?
- Represents real-world average: The most solar installations are at mid-latitudes (30°–50°), where AM is ~1.5 for much of the day. Even my solar panels are installed at the latitude ≈ of 28.665° N (close to 30°).
- Ensures uniformity: Without a standard spectrum, comparing panel efficiency would be impossible.
- Under AM1.5, the solar irradiance reaching the Earth’s surface is about 1000 W/m².
- Includes diffuse light (DHI): Unlike AM1.5D (direct beam only), AM1.5G accounts for scattered sky radiation — closer to reality for rooftops.
👉 In short: AM1.5 is the “golden reference condition” for the solar industry.
Role of Air Mass in Solar Energy
i) Spectrum Shift: Higher AM (when the sun is lower in the sky) filters out more blue/UV light.
Therefore, solar panels receive relatively more red/infrared, → affects module efficiency differently by technology.
ii) Irradiance Reduction:
- AM1 → ~1,000 W/m² at sea level.
The solar constant outside the atmosphere (AM0) is ≈ 1361 W/m². Surface irradiance at AM=1 depends on atmospheric clarity (aerosols, water vapour) and is not a fixed value. The commonly quoted 1000 W/m² is the industry testing reference under AM1.5G (STC), not an intrinsic AM1 value.
- AM2 → significantly lower, sometimes <700 W/m².
iii) Panel Efficiency Impact
Some solar technologies (like amorphous silicon) perform better under diffused red light.
Crystalline silicon is optimized for the AM1.5 spectrum.
Want to Know How Sunlight in Your Region Affects Solar Payback
The amount of sunlight falling on your solar panels is influenced by the Air Mass and the Peak Sun Hours, directly impacting your Solar ROI.
Use my Solar Feasibility Spreadsheet to find the right system size, design, and payback period based on your own location.
iv) PV spectral response
Different solar technologies respond differently across wavelengths. Crystalline silicon cells are more sensitive to the visible–near-IR region, so losing some UV/blue has a modest effect; some thin-film cells may react differently. This creates a spectral mismatch between lab test conditions (AM1.5) and real field conditions (higher AM times).
Design Implications System designers must account for seasonal changes in AM (especially in higher latitudes).
Example: India Context
India lies between 8°–35° latitude, so the Sun often stays high in the sky:
- Typical AM in India (noon): ~1.2–1.5
- Winter mornings/evenings: AM rises to 2–3 → lower irradiance.
- Summer afternoons: AM close to 1 → highest production.
👉 This explains why solar output peaks in summer noon and dips during winter mornings/evenings — even under clear skies.
For Delhi/NCR specifically:
- With ~5.25 peak sun hours daily, the AM spectrum is close to AM1.5 for most of the productive hours.
- This makes panel testing under AM1.5 highly relevant for Delhi/NCR homeowners.
I calculated the solar air mass (AM) for my location (latitude ≈ 28.665° N, longitude ≈ 77.349° E) at 12:00 PM local time (IST) on October 4, 2025.
Result (quick)
- Solar elevation (altitude) at 12:00 IST — 56.79°.
- Solar zenith angle = 90° − elevation = 33.21°.
- Air Mass (simple secant) = 1 / cos(zenith) = ≈ 1.20 (more exactly 1.195).
I used the simple secant method (AM = 1 / cos(zenith)) — this is accurate for zenith angles like ~33° (no need for more complex corrections).
Would my solar panels be producing more output? We will see in the next section.
The estimated average AM range of a few Indian cities during mid-day hours, using solar geometry:
| City (state, region) | Latitude (°N) | AM (1/cos(lat)) |
|---|---|---|
| New Delhi (N) | 28.6448° | 1.139 |
| Chandigarh (N) | 30.7333° | 1.163 |
| Srinagar (far N) | 34.0837° | 1.207 |
| Jaipur (Rajasthan) | 26.9124° | 1.121 |
| Lucknow (UP) | 26.8467° | 1.121 |
| Patna (Bihar) | 25.5941° | 1.109 |
| Ahmedabad (Gujarat) | 23.0225° | 1.087 |
| Bhopal (Madhya Pradesh) | 23.2599° | 1.088 |
| Raipur (Chhattisgarh) | 21.2514° | 1.073 |
| Bhubaneswar (Odisha) | 20.2961° | 1.066 |
| Mumbai (Maharashtra, west coast) | 19.0760° | 1.058 |
| Hyderabad (Telangana) | 17.3850° | 1.048 |
| Panaji (Goa) | 15.4909° | 1.038 |
| Chennai (Tamil Nadu) | 13.0674° | 1.027 |
| Bengaluru (Karnataka) | 12.9724° | 1.026 |
| Thiruvananthapuram (Kerala, far S) | 8.5241° | 1.011 |
| Shimla (Himachal — hill capital) | 31.1048° | 1.168 |
Quick takeaway (by region)
- Southern India (coastal/tropical) — very small AM at noon on equinox, roughly AM ≈ 1.01–1.06 (closer to AM1).
- Central India — moderate, roughly AM ≈ 1.06–1.10.
- Northern India — higher, roughly AM ≈ 1.11–1.21 (higher latitude → longer path through atmosphere at noon on equinox).
- For routine PV checks, you can expect the solar noon AM to vary roughly from ~1.01 (far south) to ~1.21 (far north) on an equinox.
Why AM = 1 doesn’t always mean higher output — the role of spectral gain vs. temperature loss
AM1 vs AM1.5 (spectral / irradiance): when the sun is higher (AM ≈ 1), the direct beam usually carries slightly more total solar irradiance or Peak Sun Hours and relatively more short-wavelength (blue/UV) light than at AM1.5. This tends to increase short-circuit current (Isc) and can slightly raise the module’s instantaneous power if everything else remains equal.
Temperature effect (real world): Solar panels under a stronger midday sun typically get hotter. Silicon modules have a negative temperature coefficient (e.g., −0.35 %/°C) — meaning that for each °C rise above the reference temperature, the maximum power drops by ~0.35%. In practice, the temperature penalty often offsets or exceeds the small spectral/irradiance gain from AM1.
Net result: the field power at AM1 can be higher, similar, or lower than at AM1.5, depending mainly on how much module temperature rises. Spectral gains are usually a few percent at best; temperature losses can be several percent for moderate temperature rises.
Step-by-step numeric example (clear math)
Baseline (STC / datasheet):
- Irradiance = 1000 W/m², Module power at STC (Pmax) = 400 W
- Reference temperature = 25 °C (STC)
- Temperature coefficient of Pmax = −0.35 % / °C → as decimal: −0.0035 / °C
Assumed AM1 condition (simple example):
- Irradiance rises to 1050 W/m² (a +5% more than 1000 W/m² because the sun is higher)
- Module temperature increases by ΔT relative to STC (we show several ΔT cases)
Formula used:
P= Pmax × (G/1000)×(1+ temp. coeff. of power × ΔT)
where G is irradiance (1050 W/m²), and the temperature coefficient = −0.0035.
| ΔT (°C) | Combined Power P (W) | % change vs STC (400 W) | Notes |
|---|---|---|---|
| 5 | 412.65 W | +3.16 % | Irradiance gain (+5%) wins over a small temperature penalty (−1.75%). |
| 10 | 405.30 W | +1.33 % | Small net gain: +5% irradiance vs −3.5% temperature loss. |
| 15 | 397.95 W | −0.51 % | Net slight loss: temperature penalty (−5.25%) ≈ , irradiance gain (+5%). |
| 20 | 390.60 W | −2.35 % | Temperature effect dominates (−7%), so output drops despite higher irradiance. |
Short interpretation/takeaway
- If module heating is small (ΔT ≤ ~10°C) at midday, AM1’s higher irradiance/spectral content will usually give a small net increase in output vs AM1.5.
- If module heating is large (ΔT ≥ ~15°C), the temperature penalty can erase the spectral/irradiance advantage and reduce power.
- Practical lesson for installers/owners: To capture the AM1 advantage, optimize cooling/ventilation, mounting height, and avoid hot back sheet installations. Also, choose modules with a low (less negative) temperature coefficient.
Conclusion
Air Mass may sound like a technical detail, but it is at the heart of solar energy science. It explains why panels generate more at noon, less in the morning, and why all solar panels are tested under the AM1.5 spectrum.
For homeowners, it simply means your solar panels are already optimized for the sunlight conditions you get daily. For students and engineers, understanding Air Mass is key to mastering PV system design and performance analysis.
In short, Air Mass bridges the gap between sunlight in space and the electricity on your rooftop.
Frequently Asked Questions: Air Mass
Q1: What is AM1.5 in solar energy?
It’s the standard air mass condition (Sun at 48.2° from zenith) used to test solar panels worldwide.
Q2: Why is Air Mass important for solar panels?
Because it affects both the spectrum and amount of sunlight reaching panels, directly influencing efficiency and energy yield.
Q3: Does Air Mass affect solar output in India?
Yes. India mostly experiences AM1.2–1.5 at noon, which is ideal. But mornings/evenings (AM2–3) reduce irradiance and energy generation.
Disclaimer:
This article is for educational use. Real project design and feasibility should be verified by licensed PV professionals.
References
- ASTM G173-03(2020), Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface, ASTM International. Available at: https://www.astm.org/g0173-03r20.html
- NREL (2020), Reference Air Mass 1.5 Spectra (AM1.5G and AM1.5D), U.S. Department of Energy, Golden, CO. Available at: https://www.nrel.gov/grid/solar-resource/spectra.html
- Kasten, F. & Young, A.T. (1989), “Revised Optical Air Mass Tables and Approximation Formula,” Applied Optics, 28(22), 4735–4738.
- NIWE / MNRE, Wind and Solar Resource Maps / Solar Resource Portal, National Institute of Wind Energy, Government of India. Available at: https://maps.niwe.res.in/resource_map/map/solar/
