Solar Panel Output Calculator with Temperature Coefficient

Inputs

Panel STC Power (Wp)

Number of Panels

Irradiance (W/m²)

Peak Sun Hours (hours/day)

Ambient Temperature (°C)

NOCT (°C) — panel temp reference

Temperature Coefficient (% per °C)

Use negative values for typical power loss (e.g. -0.35 means −0.35%/°C).

System Losses (%)

Cabling, inverter, soiling, mismatch (applied to array power).

Formulas:

Estimated cell temperature: Tcell = Tamb + (NOCT - 20) * (Irradiance / 800)
Temperature factor: (1 + tempCoeff/100 * (Tcell - 25))
Panel power: P = P_STC * (Irradiance / 1000) * temperature factor
Array power = P × number of panels × (1 - systemLosses)

Why NOCT matters

NOCT gives a practical way to estimate how hot the panel operating temperature becomes compared to ambient under typical conditions.

Tips

Mounting, ventilation and tilt affect temperature. Cooler panels produce noticeably more power — consider spacing and airflow.

Understanding Solar Panel Output and Temperature Effects

Solar panel power ratings are given under Standard Test Conditions (STC), defined as 1000 W/m² solar irradiance, 25°C cell temperature, and AM1.5 spectrum. However, real-world conditions rarely match these lab settings — especially temperature. As solar cells heat up, their voltage drops, causing overall power output to decrease.

What Is the Temperature Coefficient?

The temperature coefficient (usually between −0.3% and −0.5% per °C) describes how much the panel’s power output changes for each degree Celsius difference from 25°C. For example, a −0.35%/°C coefficient means that for every 1°C rise above 25°C, panel power falls by 0.35%. This factor becomes important in hot climates where cell temperatures often exceed 50°C.

How Cell Temperature Is Estimated (Using NOCT)

The Nominal Operating Cell Temperature (NOCT) helps estimate actual cell temperature under typical outdoor conditions. Manufacturers provide NOCT as the expected cell temperature when the panel operates at 800 W/m² irradiance, 20°C ambient air temperature, and 1 m/s wind. A common approximation is:

Tcell = Tambient + (NOCT − 20) × (Irradiance / 800)

This means that as irradiance increases or airflow decreases, the panel runs hotter, which in turn reduces its effective power.

Effect on Array Output

Since power loss applies per panel, large arrays experience amplified effects. For example, a 10-panel 400 W system with a −0.35%/°C coefficient operating at a 60°C cell temperature (35°C above STC) would lose roughly 12% of its rated power. That means a 4000 W array might only produce about 3520 W under those hot conditions.

Reducing Temperature Losses

Improve ventilation: Mount panels a few inches above the roof for airflow behind them.

Use light-colored mounting surfaces: Reflects heat and keeps modules cooler.

Choose lower temperature coefficient panels: Premium or N-type cell panels often perform better in heat.

Optimize tilt angle: Panels tilted toward the sun tend to shed heat faster than flat-mounted ones.

Real-World Implications

In warm climates, average module temperatures can exceed 50°C for much of the year. Accurately estimating performance losses helps with system sizing, inverter matching, and predicting energy yield. Temperature loss is one of the main reasons why measured system output is typically 10–15% lower than the nameplate rating.

Summary

Power decreases roughly 0.3–0.5% per °C above 25°C.

Use NOCT to estimate real cell temperatures under field conditions.

Hotter panels mean lower voltage and reduced efficiency.

Design for airflow and select efficient panels to minimize losses.

This calculator helps visualize how irradiance, ambient temperature, and NOCT combine to affect your solar array’s output — a key factor in realistic solar performance estimates.