A solar panel temperature coefficient tells you how much power a panel loses for every 1°C its cells rise above the 25°C test rating. A typical modern residential panel is about -0.30% to -0.35% per °C; premium panels reach roughly -0.25%, and older or budget panels run near -0.45%. A number closer to zero is better, and it matters most in hot climates because rooftop cells run 20°C to 25°C hotter than the surrounding air.
When you compare solar panels, most of the datasheet is about how much power they make on a cool, sunny lab day. The solar panel temperature coefficient is the one spec that tells you what happens on a real roof in July, when the panels are baking. This page explains what the number means, what counts as a good value in 2026, how to read it off a datasheet, and how much heat actually costs you in production. If you are still choosing hardware, we also cover which panels tend to have the best coefficients over on our guide to the best solar panels for homes.
What the temperature coefficient actually measures
It is the percentage of rated power a panel loses for each degree Celsius above 25°C. Panels are rated at Standard Test Conditions (STC), which fix the cell temperature at 25°C. Out in the sun, cells run far hotter than that, so the panel makes less than its nameplate rating almost any time it is producing meaningful power. The coefficient, written as a negative percentage per °C on the datasheet (the spec you want is the one for Pmax, or maximum power), is how you predict that drop.
The physics is simple: in a silicon cell, higher temperature raises internal resistance and lets more electrons recombine before they reach the circuit, so voltage and power fall as the panel heats up (ScienceDirect, temperature effects on PV performance). This is why panels quietly outperform on a cold, bright winter day and sag on a 100°F afternoon, even though the sun feels stronger in summer.
According to MySolarFY’s analysis (August 2026), a mainstream -0.35% per °C panel on a 40°C (104°F) Phoenix afternoon runs about 40°C above its 25°C rating and loses roughly 14% of its rated power at that moment. A premium -0.26% per °C panel loses about 10% under the same heat, so the better coefficient buys back 3 to 4 percentage points of output when it is hottest.
Method: (cell temp – 25°C) × coefficient, using a ~25°C cell-over-ambient rise for a roof-mounted array. For scale, our NREL PVWatts v8 model of a 6 kW system in Phoenix estimates 10,644 kWh per year (20.3% capacity factor), a figure that already bakes in these summer heat losses across the year.
What counts as a good temperature coefficient in 2026
Closer to zero is better. A panel at -0.25% per °C loses less power per degree than one at -0.40%, so it holds more of its rating on hot days. Here is how the common panel types stack up, based on current manufacturer datasheets and independent testing.
| Panel type | Typical Pmax coefficient | What it means for you |
|---|---|---|
| HJT (heterojunction, premium) | -0.24% to -0.27% per °C | Best heat tolerance; loses the least on hot roofs |
| TOPCon (modern n-type) | -0.29% to -0.32% per °C | Excellent, now common on quality residential jobs |
| Mono PERC (mainstream) | -0.34% to -0.38% per °C | Solid and by far the most widely installed today |
| Older mono / polycrystalline | -0.40% to -0.45% per °C | Larger hot-weather losses; mostly phased out |
A rule of thumb: anything at or below -0.30% per °C is very good, around -0.35% is normal and fine for most homes, and worse than -0.40% is worth a second look if you live somewhere hot. Across a dataset of more than 13,000 modules, coefficients span roughly -0.22% to -0.50% per °C, so the spread between the best and worst panels is real but not enormous. It matters more in Phoenix or Las Vegas than in Seattle.
How to read it off a datasheet
Look in the “Temperature Characteristics” box, and use the Pmax line. A datasheet lists three coefficients: one for maximum power (Pmax), one for open-circuit voltage (Voc), and one for short-circuit current (Isc). For comparing how much energy a panel will make in the heat, the Pmax coefficient is the one that counts. The Voc coefficient matters to your installer for string sizing in cold weather, but it is not the number you use to judge summer output.
You will also see NOCT (Nominal Operating Cell Temperature) on the sheet, usually 40°C to 48°C. That is the estimated cell temperature under mild, breezy test conditions, and it is a reminder that “operating” temperature is well above the 25°C rating even before a heat wave. Pair a good coefficient with real-world care: keeping airflow under the array and the glass clean both help, which we cover in simple habits to keep panels efficient.
Cell temperature vs. power loss, at a glance
Here is what a mainstream -0.35% per °C panel loses at different cell temperatures, using the simple rule (cell temperature minus 25°C, times the coefficient). Real cell temperature is usually 20°C to 25°C above the surrounding air on a sunny roof.
| Cell temperature | Degrees above 25°C rating | Approx. power loss at -0.35%/°C |
|---|---|---|
| 25°C (lab rating) | 0°C | 0% (full rated output) |
| 45°C (mild sunny day) | 20°C | about 7% |
| 55°C (warm afternoon) | 30°C | about 10.5% |
| 65°C (hot roof) | 40°C | about 14% |
How much does heat really cost you?
Over a full year, temperature losses are real but modest, because your panels are only cell-hot for part of the day. The coefficient describes a worst-case afternoon, not the annual average. Production models like NREL PVWatts already account for temperature across the whole year, which is why a sunny, hot market like Phoenix still posts strong annual output despite big midday derating. The coefficient is most useful as a tie-breaker: when two panels cost about the same and make similar power at STC, the one with the better temperature coefficient will out-produce the other every hot afternoon for 25 years.
That long horizon is where the difference adds up. A better coefficient means slightly more energy on your hottest, highest-bill days, year after year. Whether that swings the overall math for your home depends on your roof, your rates, and your climate, which is the bigger question we walk through in are solar panels worth it. You can also browse the full library of buying guides in our solar resources hub.
Temperature coefficient vs. the other specs that get confused with it
The temperature coefficient is easy to mix up with two other things on a quote, so here is the clean separation. Efficiency (often 20% to 23% today) is how much of the sunlight hitting the panel becomes electricity at STC; it does not tell you anything about heat behavior. The degradation rate (usually about 0.4% to 0.55% per year) is how much a panel permanently loses as it ages, and it is backed by the performance warranty, not the temperature spec. Heat stress can influence long-term durability, so the two are related, but they are different guarantees. For how those long-term promises are actually written, see understanding solar warranties. Because good panels and a good inverter work as a system, it is also worth reading up on choosing a solar inverter so heat-tolerant panels are not held back by weak electronics.
Does the federal tax credit change any of this?
Not the physics, but it changes the money math for a 2026 purchase, so it is worth stating plainly. The 30% federal Residential Clean Energy Credit (Section 25D) ended for expenditures made after December 31, 2025 (IRS, as of January 2026), so a homeowner who buys panels with cash or a loan in 2026 cannot claim it. That makes squeezing more lifetime output out of your hardware, including picking a good temperature coefficient, matter a little more than it did when the credit softened the upfront cost. We keep the current federal picture in our guide to the federal solar tax credit. MySolarFY does not provide tax advice; confirm your own situation with a tax professional.
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Frequently asked questions
What is a good solar panel temperature coefficient? Closer to zero is better. A modern residential panel around -0.30% to -0.35% per °C is normal and fine for most homes. Premium panels reach about -0.24% to -0.27% per °C, while older or budget panels run -0.40% to -0.45%. Below -0.30% is very good, and the difference matters most in hot climates where rooftop cells run well above the surrounding air temperature.
What does a temperature coefficient of -0.35% per degree C mean? It means the panel loses 0.35% of its rated power for every 1°C its cells rise above the 25°C test rating. If the cells reach 65°C on a hot roof, that is 40°C above rating, so the panel is producing about 14% less than its nameplate at that moment. When the cells cool back toward 25°C, the panel returns to full rated output.
Why do solar panels lose efficiency when they get hot? In a silicon cell, higher temperature raises internal resistance and lets more electrons recombine before they reach the circuit, which lowers voltage and power. The panel still works, it just converts less of the sunlight into electricity. This is a temporary loss tied to temperature, separate from the small permanent degradation a panel experiences as it ages.
Do solar panels work better in cold weather? Yes, on a per-degree basis. A cold, bright winter day can push a panel slightly above its nameplate rating because the cells are below 25°C. You usually make less total energy in winter because the days are shorter and the sun is lower, but each panel is more efficient when it is cold, which is the flip side of the temperature coefficient.
How much power do panels lose in summer heat? At the hottest part of a summer afternoon, a mainstream panel can lose roughly 10% to 15% of its rated power while the cells are baking. Across a full year the impact is much smaller, because panels are only cell-hot for part of the day, and production models like NREL PVWatts already fold these losses into their annual estimates.
References
ScienceDirect, temperature effects on PV performance
