What is temperature coefficient
Temperature coefficient is the percentage change in a solar module’s electrical parameters per degree Celsius change in operating temperature above Standard Test Conditions (STC = 25°C cell temperature). Three coefficients are reported in module datasheets:
- Voc temperature coefficient (β_Voc or simply β): change in open-circuit voltage. Typically -0.27 to -0.30%/°C for PERC.
- Isc temperature coefficient (α_Isc): change in short-circuit current. Slightly positive, typically +0.04 to +0.06%/°C.
- Pmax temperature coefficient (γ_Pmax): change in maximum power. Typically -0.34 to -0.37%/°C for PERC.
The Pmax coefficient is the most important for yield calculation. A PERC module at 60°C cell temperature (35°C above STC) loses approximately 35 × 0.36% = 12.6% power compared to STC. In Indian summer conditions, this loss is the dominant performance derate, explained in detail in why solar inverters produce less in Indian summer.
For Indian solar, temperature coefficient is critical because rooftop and ground-mount panels regularly reach 60-80°C in summer. Cell technologies with better coefficients (HJT, TOPCon) provide measurable yield advantages in hot climates.
Why temperature coefficient matters
Hot climate yield. Indian summer cell temperatures reach 75-80°C; lower coefficient means less yield loss.
Cell technology selection. HJT’s superior coefficient and higher module efficiency justify a premium in hot regions.
String sizing. Voc coefficient determines cold-morning Voc derate.
Yield modelling. Required input for accurate solar yield simulations, including tools such as SurgePV’s solar simulation software.
Lifecycle yield. Cumulative impact over 25 years is significant.
Temperature coefficient by cell technology
| Cell | Voc coefficient | Pmax coefficient |
|---|---|---|
| PERC | -0.27 to -0.30%/°C | -0.34 to -0.37%/°C |
| Mono-PERC bifacial | -0.27 to -0.30%/°C | -0.34 to -0.36%/°C |
| TOPCon | -0.25 to -0.28%/°C | -0.30 to -0.33%/°C |
| HJT | -0.21 to -0.25%/°C | -0.24 to -0.28%/°C |
| IBC | -0.27 to -0.30%/°C | -0.27 to -0.30%/°C |
| Thin film CdTe | -0.22 to -0.28%/°C | -0.21 to -0.25%/°C |
Technical deep dive
Power loss vs temperature (PERC γ = -0.36%/°C)
| Cell temperature | Loss vs STC |
|---|---|
| 25°C (STC) | 0% |
| 35°C | 3.6% |
| 45°C | 7.2% |
| 55°C | 10.8% |
| 65°C | 14.4% |
| 75°C | 18.0% |
| Item | Value (%) |
|---|---|
| 25°C | 0% |
| 35°C | 3.6% |
| 45°C | 7.2% |
| 55°C | 10.8% |
| 65°C | 14.4% |
| 75°C | 18% |
HJT vs PERC at high temperature
At 60°C cell temperature:
- PERC (γ = -0.36%/°C): 35 × 0.36 = 12.6% loss
- HJT (γ = -0.26%/°C): 35 × 0.26 = 9.1% loss
- HJT advantage: 3.5 percentage points
Across Indian summer (≈800 hours at cell temp > 60°C):
- HJT delivers 2-4% more annual yield
NOCT-based estimation
NOCT (Nominal Operating Cell Temperature) is the cell temperature at 800 W/m², 20°C ambient, 1 m/s wind.
Cell temperature approximation: T_cell = T_ambient + (NOCT − 20) × (Irradiance / 800)
For NOCT = 45°C and Indian summer ambient 40°C with 1,000 W/m²: T_cell = 40 + (25) × (1000/800) = 40 + 31 = 71°C
Test method (IEC 61853)
Module measured at multiple temperatures (25°C, 50°C, 75°C) and irradiance levels (1000, 800, 500, 200 W/m²). Coefficients extracted by linear fit.
Real-world applications
PVsyst yield modelling input.
String sizing cold/hot calculations.
Module selection for hot climates, part of a broader strategy to maximise solar output in Indian conditions.
Bankable yield assessment.
Best practices
Use module datasheet values for yield modelling.
Verify IEC 61853 measurement test.
Account for temperature derating in PR.
For hot Indian sites, prefer TOPCon or HJT.
Document temperature coefficient assumption in yield models.
Indian market context
Indian summer cell temperatures: 65-80°C common.
Temperature derate dominates in mid-day yield modelling.
HJT premium increasingly justified in dry hot regions (Rajasthan, Gujarat), where detailed rooftop engineering design accounts for elevated cell temperatures.
ALMM-listed modules report standard coefficients.
Standards and certifications
| Standard | Scope |
|---|---|
| IEC 61853 | Module performance measurement |
| IEC 61215 | Module qualification |
| IS 14286 | Indian module standard |
| Manufacturer datasheets | Specific values |
Key takeaways
Temperature coefficient measures power loss per degree of cell temperature rise above STC. Critical for Indian climates where cell temperatures regularly reach 60-80°C. PERC: -0.34 to -0.37%/°C. TOPCon: -0.30 to -0.33%/°C. HJT: -0.24 to -0.28%/°C. HJT’s superior coefficient provides 2-4 percent more annual yield in hot Indian conditions, often justifying its price premium.
Further reading
For how Temperature Coefficient plays out in real projects, these guides go deeper:
Sources
- IEC 61853, Module performance measurement.
- IEC 61215, Module qualification.
- IS 14286, Indian module standard.
- ITRPV Roadmap, latest edition.
- Manufacturer datasheets from major module brands.
- NREL Module Temperature Studies.
- Sandia National Laboratories module temperature reports.
- IEA PVPS Task 13 Reports.
- Duffie J. A., Beckman W. A., Solar Engineering of Thermal Processes, Wiley.
Frequently Asked Questions
What is temperature coefficient in simple words?
What are typical Pmax coefficients?
Why does temperature reduce output?
How hot do solar panels get?
What is Voc coefficient?
Is Isc coefficient positive or negative?
How is power loss calculated?
Does HJT beat PERC in heat?
Is bifacial coefficient same as monofacial?
Does temperature coefficient improve with new cells?
How is coefficient measured?
Does coefficient depend on irradiance?
Are bifacial back side coefficients same?
Does coefficient affect string sizing?
Is lower coefficient worth premium?
From the Blog
Co-founder & CEO driving Qbits Energy's product strategy and growth across India.