Temperature Coefficient of Pmax Explained: How Heat Reduces Solar Panel Output

The temperature coefficient of Pmax (gamma) tells you exactly how much power a solar panel loses for each degree Celsius the cells heat above 25°C. A typical value of -0.35%/°C means a 400W panel loses 1.4W for every degree above STC reference temperature. On a hot summer day when cell temperatures hit 65°C, that is a 14% power loss — 344W instead of 400W. This coefficient varies significantly by cell technology, from -0.38%/°C for PERC to -0.26%/°C for HJT, making it one of the most important specs for hot climate installations.

Why temperature kills solar panel output

Solar cells convert sunlight to electricity by exploiting the bandgap of silicon — the energy threshold that incoming photons must exceed to knock electrons free and generate current. This bandgap is temperature-sensitive. As silicon heats up, the bandgap narrows, which has two competing effects on the cell's electrical output.

Current increases slightly. A narrower bandgap means more photons have enough energy to generate electron-hole pairs. Short-circuit current (Isc) rises by about +0.04 to +0.06%/°C, a modest gain.

Voltage decreases significantly. The narrower bandgap also increases the intrinsic carrier concentration in the silicon, which reduces the built-in voltage of the p-n junction. Open-circuit voltage (Voc) drops by about -0.27 to -0.35%/°C, a substantial loss.

Since power equals voltage times current (P = V x I), the large voltage drop overwhelms the small current gain, producing a net power decrease of -0.30 to -0.45%/°C for crystalline silicon cells.

Temperature coefficients by cell technology

Cell technology	Typical coefficient	400W panel output at 65°C cell temp	Annual loss vs HJT (hot climate)
HJT	-0.24 to -0.26%/°C	358-362W	Baseline
TOPCon	-0.29 to -0.34%/°C	346-354W	1-2%
Mono-PERC	-0.34 to -0.38%/°C	339-346W	2-4%
Polycrystalline	-0.40 to -0.45%/°C	328-336W	4-6%
Thin-film (CdTe)	-0.20 to -0.22%/°C	365-368W	Better than HJT

The calculation for the 65°C column: Temperature rise above STC = 65 - 25 = 40°C. Power loss = coefficient x 40°C. For PERC at -0.35%/°C: loss = 14%, output = 400 x 0.86 = 344W.

HJT panels achieve their superior temperature coefficient because of the amorphous silicon (a-Si) layers that sandwich the crystalline silicon wafer. Amorphous silicon has a wider bandgap (~1.7 eV vs 1.12 eV for crystalline silicon), which reduces the voltage drop at elevated temperatures. TOPCon is better than PERC because the tunnel oxide passivation reduces recombination losses that would otherwise worsen at high temperatures.

Worked example: Phoenix vs Portland

To see how much the temperature coefficient matters in practice, compare two 8kW systems — one in Phoenix, Arizona and one in Portland, Oregon — using PERC panels (-0.35%/°C) vs HJT panels (-0.26%/°C).

Phoenix summer peak: 42°C ambient, 1000 W/m², NOCT = 44°C. Cell temp = 42 + (44-20) x 1.25 = 72°C. Temperature rise = 47°C above STC.

PERC loss: 47 x 0.35% = 16.5%. Output: 8kW x 0.835 = 6,680W
HJT loss: 47 x 0.26% = 12.2%. Output: 8kW x 0.878 = 7,024W
Difference: 344W (5.1%) in favor of HJT at peak

Portland summer peak: 28°C ambient, 900 W/m², NOCT = 44°C. Cell temp = 28 + (44-20) x 1.125 = 55°C. Temperature rise = 30°C above STC.

PERC loss: 30 x 0.35% = 10.5%. Output: 8kW x 0.895 = 7,160W
HJT loss: 30 x 0.26% = 7.8%. Output: 8kW x 0.922 = 7,376W
Difference: 216W (3.0%) in favor of HJT at peak

The temperature coefficient advantage of HJT is nearly twice as large in Phoenix as in Portland. Over a full year, the annual energy production advantage of HJT over PERC is roughly 3-4% in Phoenix and 1.5-2.5% in Portland.

How temperature coefficient is measured

The temperature coefficient is measured per IEC 61215 and IEC 60891. The panel is placed in a temperature-controlled chamber and illuminated with a calibrated solar simulator. Starting at 25°C, the panel's full IV curve is measured. The chamber temperature is then increased in steps (typically to 50°C and 75°C), and the IV curve is measured at each temperature.

The temperature coefficient of Pmax is the slope of the line fitting Pmax vs temperature, expressed as a percentage change per degree Celsius relative to the 25°C value. The measurement is taken at constant irradiance (1000 W/m²), isolating the temperature effect from irradiance variation.

Most datasheets report a single coefficient value, but in reality the relationship is not perfectly linear. The coefficient is slightly more negative at lower irradiances and slightly less negative at higher temperatures. For system design purposes, the single datasheet value is accurate enough.

Using the temperature coefficient for system sizing

The temperature coefficient is not just an academic number — it directly affects your system size and inverter selection.

Array sizing: If you need 8kW of AC power during summer peak in a hot climate, you cannot simply install 8kW of STC-rated panels. You need to oversize the array to compensate for temperature losses. With PERC panels losing 14-17% to temperature, you need roughly 9.3-9.6kW of STC capacity to deliver 8kW at peak summer temperatures.

Inverter clipping: Conversely, on cold sunny days in winter, panels produce near or above their STC rating (because cell temperatures may be close to 25°C). If you have oversized your array for summer heat, your winter peak output will exceed the inverter's rated capacity, causing clipping. Proper system design balances these seasonal extremes.

Related terms

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Frequently Asked Questions

What is the temperature coefficient of Pmax?

The temperature coefficient of Pmax, symbolized by the Greek letter gamma, is the percentage change in a solar panel's maximum power output for each degree Celsius the cell temperature rises above the 25°C STC reference temperature. It is always a negative number because power decreases as temperature increases. A coefficient of -0.35%/°C means the panel loses 0.35% of its rated power for every 1°C above 25°C. It is listed in the thermal characteristics section of every solar panel datasheet.

What is a good temperature coefficient for a solar panel?

Lower absolute values (closer to zero) are better because they mean less power loss from heat. HJT panels have the best coefficients at -0.24 to -0.26%/°C. TOPCon panels are next at -0.29 to -0.34%/°C. Mono-PERC panels are -0.34 to -0.38%/°C. Older polycrystalline panels are worst at -0.40 to -0.45%/°C. For hot climates like Arizona, Texas, or Florida, the difference between a -0.26%/°C HJT panel and a -0.38%/°C PERC panel can mean 3-4% more annual energy production — a meaningful advantage over 25 years.

How do I calculate power loss from temperature?

Use this formula: Power loss (%) = temperature coefficient x (cell temperature - 25°C). For example, a 400W panel with a coefficient of -0.35%/°C on a day when cell temperature is 60°C. The temperature rise is 60 - 25 = 35°C. Power loss = -0.35% x 35 = -12.25%. Actual power = 400W x (1 - 0.1225) = 351W. To estimate cell temperature, use: Cell temp = Ambient + (NOCT - 20) x (Irradiance / 800). At 35°C ambient, 1000 W/m², and NOCT of 44°C, cell temp = 35 + 30 = 65°C.

Why does solar panel power decrease when temperature increases?

As temperature rises, the silicon bandgap energy decreases, which increases current slightly but decreases voltage significantly. The voltage drop is much larger than the current gain, so net power drops. Specifically, open-circuit voltage (Voc) decreases by about -0.27 to -0.35%/°C, while short-circuit current (Isc) increases by only about +0.04 to +0.06%/°C. Since power equals voltage times current, the large voltage decrease dominates, producing a net power loss of -0.30 to -0.45%/°C depending on the cell technology.

Which solar panel technology has the best temperature coefficient?

HJT (Heterojunction Technology) panels have the best temperature coefficients in mass production, typically -0.24 to -0.26%/°C. This is because HJT cells use amorphous silicon layers that have a wider bandgap, reducing the voltage loss at elevated temperatures. TOPCon panels are second best at -0.29 to -0.34%/°C. Thin-film CdTe (cadmium telluride) panels also perform well at -0.20 to -0.22%/°C, though their lower baseline efficiency limits their appeal for residential rooftops.

How much does temperature coefficient matter for my location?

It depends on how hot your area gets. In a cool climate like Seattle or Portland where summer cell temperatures rarely exceed 55°C, the difference between a -0.26%/°C HJT panel and a -0.35%/°C PERC panel is only about 2.7% power difference at peak. In Phoenix where cell temperatures can reach 75°C, the difference is 4.5%. Over 25 years, that 4.5% annual advantage can mean thousands of dollars in additional energy production, potentially justifying the higher upfront cost of HJT panels in hot climates.

Does the temperature coefficient change as panels age?

Temperature coefficients are generally stable over the panel's lifetime. The physics of the bandgap-temperature relationship in silicon does not change significantly with aging. However, as panels degrade (losing 0.4-0.55% of power per year due to other mechanisms like LID, PID, and encapsulant browning), the absolute power loss per degree becomes slightly smaller because the baseline power is lower. For practical system design purposes, the nameplate temperature coefficient is used throughout the system lifetime without adjustment.

Sources

Marko Visic

Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.