Temperature Coefficient

What temperature coefficient is

Temperature coefficient measures how much a solar panel’s electrical output changes per degree Celsius of cell temperature change. Three coefficients are reported on every panel datasheet:

Temperature coefficient of Pmax: How power output changes with temperature. Typically minus 0.25% to minus 0.45% per deg C.

Temperature coefficient of Voc: How open-circuit voltage changes with temperature. Typically minus 0.21% to minus 0.32% per deg C.

Temperature coefficient of Isc: How short-circuit current changes with temperature. Typically positive 0.04% to 0.06% per deg C.

The negative coefficients for power and voltage mean output decreases at higher temperatures. The positive coefficient for current is much smaller.

For solar plant operation in hot Indian climates, the temperature coefficient is one of the most important panel specifications. Indian summer cell temperatures reach 55 to 65 deg C, well above STC’s 25 deg C, causing 10% to 15% output loss for typical Mono PERC panels.

Why temperature affects solar cells

Several physical mechanisms cause solar cells to lose efficiency at higher temperatures:

Bandgap reduction: Silicon’s bandgap decreases at higher temperatures. The cell absorbs slightly more light (positive effect, small).

Intrinsic carrier concentration: Higher temperature increases intrinsic carriers, raising the dark current. This reduces Voc (negative effect, significant).

Carrier mobility: Higher temperature reduces carrier mobility, increasing series resistance. This affects fill factor (small negative effect).

Recombination: Higher temperature increases recombination rates throughout the cell. This affects Voc and current collection (negative effect).

The cumulative effect: cell power decreases by 0.25% to 0.45% per deg C, with the specific value depending on cell architecture.

Temperature coefficients by cell technology

The progression from older Aluminium BSF to modern HJT has reduced temperature sensitivity significantly. HJT’s minus 0.24% per deg C produces 30% to 40% less power loss in hot conditions compared to older cells.

Impact on Indian operation

For typical Indian solar conditions:

Ambient temperature in summer: 35 to 45 deg C.

Cell operating temperature (above ambient): 20 to 30 deg C above ambient.

Cell operating temperature in Indian summers: 55 to 70 deg C.

Difference from STC’s 25 deg C: 30 to 45 deg C.

Power loss at minus 0.34% per deg C: 10% to 15%.

For a 100 kWp Mono PERC plant in Ahmedabad:

STC peak output: 100 kW.

Mid-day summer output (cell at 60 deg C): 100 × (1 - 0.34% × 35) = 88.1 kW.

Loss to temperature: 11.9 kW.

For the same plant with HJT (minus 0.25% per deg C):

Mid-day summer output (cell at 60 deg C): 100 × (1 - 0.25% × 35) = 91.25 kW.

Loss to temperature: 8.75 kW.

HJT recovers 3.15 kW (3.1%) over Mono PERC at this condition.

Over a year, the HJT advantage compounds. In high-irradiance Indian locations, HJT typically delivers 4% to 6% more annual energy than Mono PERC due to better temperature behaviour.

Temperature coefficient and string design

The Voc temperature coefficient is critical for string design.

For inverter input voltage limits, cold-day Voc must stay below the inverter’s maximum.

For a 49 V STC Voc panel with minus 0.27% per deg C Voc coefficient:

At minus 5 deg C ambient (cold day, cell near ambient): Voc = 49 × (1 + 0.27% × 30) = 53.0 V.

At plus 60 deg C cell temperature: Voc = 49 × (1 - 0.27% × 35) = 44.4 V.

The string voltage range from cold to hot conditions can vary by 16% to 18%. Designs must account for both extremes.

For Indian inland locations with mild winters, cold-day Voc is the limiting factor for maximum panels per string. For hot summer operations, low Voc may reach the inverter’s MPPT minimum.

Reducing operating temperature

Several design choices reduce cell operating temperature:

Adequate air gap underneath panels: 100 mm or more clearance allows wind cooling. Reduces operating temperature by 3 to 5 deg C.

Mounting orientation: South-facing tilts allow good thermal circulation. Some east-west and flat-mounted designs trap heat.

Tracker designs: Single-axis trackers expose both sides of the panel to wind, cooling the panel more effectively than fixed-tilt.

Light-coloured mounting structures: Reflective surfaces beneath the panel reduce heat absorption.

White rooftops or ground beneath panels: Higher albedo reduces heat radiated to the panel.

Reducing operating temperature by 5 deg C recovers about 1.7% of power for Mono PERC. The cumulative effect over a year is meaningful.

Temperature coefficient in financial modelling

PVsyst, SAM, and similar tools model temperature coefficient explicitly. The site-specific temperature profile is combined with the panel’s temperature coefficients to project monthly and annual energy.

For lender-grade financial models:

Use site-specific temperature data (not generic averages).

Apply panel-specific temperature coefficients (not generic Mono PERC values).

Model NOCT (Nominal Operating Cell Temperature) characteristics in addition to STC.

Verify the choice of panel based on lifetime energy, not just upfront cost.

Common temperature coefficient mistakes

Comparing panels solely by STC nameplate. Two 540 Wp panels with different temperature coefficients produce different annual energy.

Ignoring temperature coefficient in tropical Indian climates. The effect is much larger than in temperate climates.

Treating temperature coefficient as fixed across the panel’s life. Slight degradation over years.

Confusing power coefficient with voltage coefficient. They are different numbers and serve different design purposes.

Underestimating cell temperature. Cell temperature is significantly higher than ambient.

Best practices

For hot Indian climates, prefer cell technologies with lower temperature coefficients (TOPCon or HJT over Mono PERC).

For long-term financial models, include site-specific temperature data and panel-specific coefficients.

For mounting design, allow adequate air gap underneath panels for cooling.

For inverter selection, verify that cold-day Voc stays within inverter limits using temperature coefficient.

For comparing panels, calculate expected annual energy using temperature coefficient and site temperature profile, not just STC nameplate.

Standards and references

Temperature coefficient is measured per IEC 61215 (module qualification) and IEC 61853 (energy yield characterisation). The measurement is part of standard cell and module testing in manufacturing.

Related glossary terms

• Mono PERC
• TOPCon Solar Panel
• HJT Solar Panel
• Standard Test Conditions
• Nominal Operating Cell Temperature
• Performance Ratio
• Open-Circuit Voltage
• Short-Circuit Current
• Maximum Power Point

Key takeaways

Temperature coefficient measures how solar panel output changes per degree Celsius of cell temperature change. Three coefficients are reported: Pmax (power), Voc (voltage), and Isc (current). Mono PERC typically has Pmax temperature coefficient of minus 0.34% to minus 0.37% per deg C; TOPCon improves to minus 0.29% to minus 0.32%; HJT achieves minus 0.24% to minus 0.27%. For hot Indian climates with cell temperatures of 55 to 65 deg C in summer, temperature coefficient causes 10% to 15% output loss compared to STC. Lower temperature coefficients (HJT, TOPCon) deliver meaningfully more annual energy and justify premium pricing in hot locations.