Temperature Coefficient and Solar Panels

Solar panels are tested and rated at 25°C (77°F), a temperature they almost never actually reach in real-world operation. In full sun on a summer day, panel surface temperatures commonly hit 45–65°C (113–149°F), and at those temperatures, power output drops measurably. The temperature coefficient is the number that tells you exactly how much — and understanding it helps you compare panel technologies, predict real-world performance, and make a better buying decision, especially if you live somewhere hot.

What the Temperature Coefficient Means

The temperature coefficient of power (Pmax) is expressed as a percentage per degree Celsius above the Standard Test Condition (STC) temperature of 25°C. A typical specification looks like this: -0.35%/°C.

This means: for every degree Celsius the panel temperature rises above 25°C, the panel loses 0.35% of its rated power output. The negative sign confirms this is a loss — not a gain.

A real example: a 400 W panel with a -0.35%/°C coefficient on a day when the panel surface reaches 65°C.

• Temperature rise above STC: 65°C − 25°C = 40°C
• Power reduction: 40 × 0.35% = 14%
• Actual output: 400 W × (1 − 0.14) = 344 W

The 56 W gap isn’t a defect — it’s physics. Solar cells are semiconductor devices, and heat reduces the bandgap energy of silicon, lowering open-circuit voltage and thus power output. This happens in every silicon panel, in every climate, on every sunny afternoon.

Why Standard Test Conditions Don’t Reflect Reality

STC — 1,000 W/m² irradiance, 25°C cell temperature, AM 1.5 spectrum — is a lab condition designed to allow consistent comparison between panels, not to reflect real-world operation. A panel sitting in full sun on a rooftop in Phoenix in July isn’t running at 25°C. It’s running at 50–70°C depending on wind, mounting style, and ambient temperature.

This is why some datasheets also publish NMOT (Nominal Module Operating Temperature, formerly NOCT) ratings, which test at 800 W/m² irradiance, 20°C ambient temperature, and 1 m/s wind — closer to real-world summer conditions. NMOT power ratings are typically 10–15% lower than STC ratings. When comparing two panels, the NMOT rating is a better predictor of real-world summer afternoon output than STC.

Temperature Coefficient by Panel Technology

Not all solar cells respond to heat equally. Temperature coefficient varies significantly across panel technologies:

The practical implication: on a 65°C summer panel surface, an HJT panel loses about 10% of its rated power, while an older polycrystalline panel loses 16–20%. For a 10 kW system in Phoenix, that difference can add up to 500–1,000 kWh per year.

How Much Does Temperature Coefficient Actually Matter for Your System?

The answer depends primarily on your climate. In hot climates — Arizona, Nevada, Texas, Florida, Southern California — temperature coefficient is a significant performance differentiator. Panel temperatures in these regions frequently exceed 60°C for hours per day during summer, and summer is when the grid is most stressed and energy is most valuable.

In cooler climates — the Pacific Northwest, New England, the Great Lakes — panel temperatures rarely stay above 45°C for extended periods. Temperature coefficient still matters, but it’s less decisive. A high-efficiency PERC panel may outperform an HJT panel in overall annual output in Seattle simply because it has more rated watts per dollar, even if the HJT handles heat better.

A rough rule of thumb: if your area regularly sees ambient temperatures above 90°F (32°C) in summer, temperature coefficient should factor meaningfully into your panel selection. If your climate is mild or cool, prioritize price-per-watt and overall efficiency rating over temperature coefficient.

Temperature Coefficient of Voltage and Current

Most datasheets include three separate temperature coefficients:

Pmax (Power): The overall power coefficient. Most important for system output calculations. Range: −0.24% to −0.50%/°C depending on technology.

Voc (Open-Circuit Voltage): Voltage decreases with temperature. Typical: −0.26% to −0.35%/°C. This is critical for string inverter design — in cold weather, Voc rises, and designers must ensure string voltage doesn’t exceed the inverter’s maximum input voltage.

Isc (Short-Circuit Current): Current actually increases slightly with temperature. Typical: +0.04% to +0.08%/°C. This partially offsets the voltage loss but not nearly enough to compensate for it.

The voltage coefficient has an important cold-weather implication: on a cold winter morning at −20°C (−4°F), a panel rated at 50 Voc can produce up to 60+ Voc. String inverters have a maximum DC input voltage (typically 600V or 1000V), and if a string of panels exceeds that in cold conditions, the inverter shuts down for protection. Installers must calculate maximum string voltage using the cold-weather Voc coefficient to ensure safe system design.

How Installers Use Temperature Coefficient in System Design

Professional installers use tools like Aurora Solar or PVWatts to model a system’s expected annual output using weather data including hourly temperature distributions. Temperature coefficient is one of the derating factors automatically applied — a well-designed system model will show 85–92% of STC-rated production on an annual basis, with temperature derating typically accounting for 3–8% of the gap depending on climate.

When comparing proposals from different installers using different panels, ask for the production estimate in kWh/year — not just system capacity in kW. Two 10 kW systems using different panels in Phoenix can differ by 500–1,000 kWh/year in projected output based on panel quality and temperature coefficient alone.

Frequently Asked Questions

What is a good temperature coefficient for solar panels?

For silicon solar panels, anything below −0.35%/°C is good; below −0.30%/°C is excellent. HJT panels at −0.24%/°C to −0.26%/°C represent the best thermal performance available in commercial silicon technology. Thin-film panels (First Solar Series 6) offer −0.20%/°C, the best of any mass-produced technology, but they are primarily used in utility-scale projects. For residential use, HJT panels from REC, Panasonic, and Canadian Solar (HiHero line) offer the best temperature performance.

Does temperature coefficient affect panels in cold weather?

In cold weather, the relationship inverts: panels below 25°C actually produce slightly more than their rated output. A panel at 0°C with a −0.35%/°C coefficient produces about 8.75% more power than at STC. This is why solar panels often perform well on cold, clear winter days despite shorter daylight hours — the cold compensates somewhat for the reduced irradiance. However, the cold-weather Voc rise is a design concern that installers must account for in string voltage calculations.

Why do solar panels lose efficiency in heat?

Heat increases the thermal energy of electrons in the semiconductor material, which reduces the bandgap and lowers open-circuit voltage. Voltage is the primary driver of power loss — the current barely changes with temperature, but voltage drops measurably above 25°C. Silicon solar cells are particularly susceptible to this because silicon has a temperature-sensitive bandgap; HJT cells use thin amorphous silicon layers that reduce this sensitivity.

Do bifacal panels have better temperature coefficients?

Bifacial panels have the same temperature coefficient as their monofacial equivalents of the same cell technology — the bifacial design captures rear-side light but doesn’t change the cell’s thermal properties. A bifacial TOPCon panel has the same −0.30%/°C coefficient as a monofacial TOPCon panel. However, bifacial panels often generate more total power through rear gain, which can offset temperature losses on an annual basis in high-irradiance climates.

Should I choose HJT panels specifically for their temperature coefficient?

If you’re in a hot climate (Southwest, Southeast, Texas) and evaluating premium panels, yes — the temperature coefficient difference between HJT (−0.25%/°C) and PERC (−0.38%/°C) translates to a real production difference of 5–10% on hot afternoons. Over 25 years in Phoenix, this could mean 5,000–12,000 additional kWh from an HJT system vs. a PERC system of equivalent rated capacity. Whether the HJT cost premium is justified depends on local electricity rates and the price difference between products.

Summing Up

The temperature coefficient tells you how much power your solar panels lose when they heat up in the sun — typically −0.24% to −0.50% per degree Celsius above 25°C. For hot climates, this number matters: a panel reaching 65°C on a summer afternoon in Phoenix is delivering 10–20% less power than its rated output depending on the technology. HJT panels perform best in heat, followed by TOPCon, then PERC. In cooler climates, the difference is smaller and price-per-watt often outweighs thermal performance in the selection decision. When comparing solar quotes, always ask for annual kWh production estimates — that number captures temperature effects automatically and gives you an apples-to-apples comparison between systems using different panels.

If you’re ready to get solar quotes for your home and want to discuss which panel technology makes sense for your climate and budget, call Solar Panels Network USA at (855) 427-0058 for a free consultation.

Updated May 2026