Quick answer
Solar Panel Temperature Coefficient A solar panel temperature coefficient (of Pmax) is the percentage of rated power it loses for every degree Celsius its cells run above 25°C, the temperature at which the wattage on the label was measured.
A panel rated −0.34%/°C gives up 3.4% of its power for every 10°C of extra cell heat. The closer the number is to zero, the better the panel holds output on hot days.
Quick facts
The key facts about solar panel temperature coefficient, with sources:
- Unit
- % per °C (some sheets write % per K; the same thing)
- Reference temperature
- 25°C cell temperature, standard test conditions (STC) 1
- Range on current data sheets cited here
- −0.24%/°C (heterojunction) to −0.39%/°C (PERC solar shingle) 27
- Model defaults (PVWatts V5, 2014)
- Standard −0.47, premium −0.35, thin film −0.20 %/°C 8
- Where to find it
- Data sheet, "Temperature characteristics" block, line "Pmax" or "Pmpp"
Key takeaways
- The temperature coefficient tells you how much power a panel loses for each degree its cells run above 25°C.
- It uses cell temperature, not air temperature. Cells in sun run 20°C or more above the air.
- Current data sheets range from about −0.24%/°C (heterojunction) to −0.39%/°C (a solar shingle).
- Roof-mounted panels run hotter than open-rack ones, so the number matters more on roofs.
- Use it to compare panels in hot places. It does not predict yearly production on its own.
What the number means
Every panel is tested at 25°C cell temperature under strong lab light. That is where the watts on the label come from. Real panels rarely sit at exactly 25°C.
The temperature coefficient tells you how far output moves when cells are warmer or cooler. It is always written as a negative percent per degree, because heat lowers power.
A data sheet usually lists three coefficients: one for power (Pmax), one for voltage (Voc) and one for current (Isc). The power one is the number to compare between panels.
Sources: [1]
How it works: the formula
Power at cell temperature T = rated power × [1 + coefficient × (T − 25°C)]
The coefficient is negative, so any cell temperature above 25°C cuts output. Anything below 25°C raises it slightly.
Power falls with heat mainly because voltage drops as cells warm. The REC Alpha Pure-RX sheet shows it: voltage falls 0.24% per degree, while current rises 0.04% per degree. The small current gain cannot make up the voltage loss.
Sources: [2]
Worked example: a 400 W panel at 65°C
An example, not a forecast. A 400 W panel with the −0.34%/°C coefficient printed on the Canadian Solar HiKu6 mono PERC data sheet 1, on a summer afternoon when its cells reach 65°C.
| Step | Math | Result |
|---|---|---|
| Degrees above 25°C | 65 − 25 | 40°C |
| Power lost | 40 × 0.34% | 13.6% |
| Output at 65°C | 400 W × (1 − 0.136) | 345.6 W |
| Same panel with a −0.24%/°C coefficient (REC heterojunction figure) 2 | 400 W × (1 − 0.096) | 361.6 W |
The two panels carry the same label, but at that moment the lower coefficient gives 16 W more. The math ignores sunlight strength. Real output also depends on irradiance, which is usually below the 1,000 W/m² of the rating test.
Coefficients by panel type, from real data sheets
| Technology | Product (data sheet) | Temp. coefficient of Pmax |
|---|---|---|
| Heterojunction | REC Alpha Pure-RX 2 | −0.24%/°C |
| Back contact | Maxeon 7 3 | −0.27%/°C |
| TOPCon | Canadian Solar TOPHiKu6 4 | −0.29%/°C |
| Thin film (CdTe) | First Solar Series 7 TR1 5 | −0.32%/°C |
| Mono PERC | Canadian Solar HiKu6 1 | −0.34%/°C |
| Polycrystalline | Canadian Solar KuMax CS3U-P (2018) 6 | −0.38%/°C |
| Solar shingle (mono PERC) | GAF Timberline Solar SM210 (2021) 7 | −0.39%/°C |
The three coefficients on a data sheet
- Pmax / Pmpp coefficient (γ): power change per degree. Compare this one between panels.
- Voc coefficient (β): how fast open-circuit voltage falls with heat. Installers use it to size strings.
- Isc coefficient (α): small and positive. Current rises slightly with heat.
- NMOT or NOCT: a guide to how hot the cells run in the field, under the conditions printed beside it.
Where you find it, and where models use it
On the panel data sheet, look for a block called "Temperature characteristics" or "Temperature ratings". Your installer’s proposal should link the sheet.
Production models use it too. NREL’s PVWatts V5 manual (2014) set default coefficients by module type: −0.47%/°C for standard, −0.35%/°C for premium and −0.20%/°C for thin film. Those were model defaults for the panels of that era, not product specs. Current sheets differ, as the table above shows.
Sources: [8]
Why it matters more in the Southeast
Data sheets give a second clue: the nominal module operating temperature (NMOT). It is the cell temperature at 800 W/m² of sun, 20°C air and a 1 m/s breeze.
It is 41 ± 3°C on the HiKu6 sheet and 44 ± 2°C on the REC sheet. So cells run roughly 20 to 25°C above the air even in mild conditions.
Roof mounts run hotter still. The PVWatts V5 manual assumes about 45°C for open racks and roughly 49°C for roof mounts with 4-inch standoffs, because less air flows under the panels.
Apply that rise to a 35°C (95°F) summer afternoon in South Carolina, Georgia or Virginia. Cell temperatures in the high 50s to 60s°C are plausible. That is our estimate from the published figures, not a measurement. It is why a hot July can produce less than a cooler, clear April.
What a low coefficient does and does not do
A low coefficient helps
- Hold more output on hot afternoons.
- Narrow the summer dip in monthly production.
- Make the most difference on roofs with little airflow.
- Give a fair way to compare panels from different makers.
It does not
- Predict yearly kWh on its own.
- Make up for shade, poor tilt or a north-facing roof.
- Always justify a higher price.
- Matter much in cool, cloudy places.
Limits of the number
The coefficient is a straight-line approximation. The REC sheet notes its coefficients "are linear values". Real panels drift a little from that line at very hot or cold temperatures.
It also ignores light level, wind and dirt. For a full yearly estimate, installers use models that combine weather, tilt, shade and heat. Our guide to why systems underproduce covers the other factors.
How it affects cost and value
HyreSolar does not publish panel prices. Panels with lower coefficients, such as heterojunction and back contact, often cost more. Whether that pays depends on how many hot hours your panels see over 25 years or more.
Compare on energy. Ask each installer for yearly kWh using the actual panel, then compare the cost per kWh. Our cost per watt guide and the savings calculator below help.
How to use it when choosing panels
You do the comparing; the installer does the electrical sizing.
- Ask every installer for the panel data sheet.
- Find the Pmax temperature coefficient on each one.
- Note the NMOT or NOCT and the conditions beside it.
- Ask whether the production estimate used that panel’s coefficient and a roof-mount temperature.
- Compare yearly kWh and cost per kWh across quotes.
- Leave string sizing, which uses the voltage coefficient on cold mornings, to the licensed installer.
Heat, care and your monitoring
Expect a summer dip in midday output. Compare each month with the same month last year, not with April.
Do not hose panels down to cool them. Sudden cold water on hot glass is not recommended, and roof work is risky. Our cleaning guide covers safe care.
Keep the space under the panels clear of leaves and nests, so air can flow.
When heat points to a real problem
Normal heat loss is gradual and the same across all panels. Call a licensed installer if you see:
- One panel far below the others on a hot day.
- Scorch marks or brown spots, which can mark a hot spot.
- The inverter shutting down or derating on hot afternoons.
- A burning smell near the inverter or roof.
Rule of thumb
Each 0.1 point of coefficient is worth about 4% of power when cells run 40°C above 25°C. Compare it on every quote in a hot state.
Standards behind the figure
No code sets a maximum temperature coefficient. It is a performance figure the maker reports.
The tests behind data-sheet ratings come from panel standards. The REC sheet, for example, lists IEC 61215:2021 for design and performance, and IEC 61730:2023 and UL 61730 for safety. UL says the NEC, IBC and IRC require PV products on buildings to be certified by a Nationally Recognized Testing Laboratory.
The cold-morning voltage limit is a code matter. Installers size strings so voltage stays within the inverter and equipment ratings under the adopted electrical code.
Temperature coefficient vs related ratings
| Rating | What it tells you | Where it matters |
|---|---|---|
| Temperature coefficient (Pmax) | Power lost per °C above 25°C | Hot climates, roof mounts |
| Panel wattage | Power at 25°C and 1,000 W/m² | System size |
| Module efficiency | Watts per square metre at test conditions | Tight roofs |
| NMOT / NOCT | How hot cells run under mild field conditions | Estimating real cell temperature |
| Degradation rate | Yearly loss with age | Long-term output; see degradation |
Misconceptions
- Myth Solar panels work best in hot weather.
- Reality They make most in strong sun, but heat itself costs power. Bright, cool days are the most productive per hour of sun.
- Myth The temperature coefficient tells you yearly production.
- Reality It covers one factor. Yearly kWh depends on size, direction, shade and local sun.
- Myth A lower coefficient always justifies a pricier panel.
- Reality Only if the extra energy over the system’s life is worth more than the price gap.
- Myth The coefficient uses air temperature.
- Reality It uses cell temperature, which runs well above the air in sun 1.
Heat and panels in SC, GA and VA
Summers in all three states are long and humid, so the coefficient is worth comparing on any quote. How heat losses hit your bill depends on how your utility credits power.
South Carolina customers who applied after 1 June 2021 use a Solar Choice tariff under Act 62 10. Georgia Power caps residential renewable systems at 10 kW, so getting more kWh per kW can matter there 11.
Virginia allows residential net metering up to 25 kW on investor-owned utilities 12.
When the coefficient should sway your choice
- You live where summers are long and hot.
- Panels will sit close to the roof with little airflow.
- Two quotes are close on price and wattage; the lower coefficient breaks the tie.
- Next step: run both quotes through the savings calculator and compare 25-year kWh.
Related guides and tools
Questions about solar panel temperature coefficient
What is a good temperature coefficient for a solar panel?
On the current data sheets cited here, −0.24%/°C (heterojunction) to −0.29%/°C (TOPCon) is the better end. About −0.34%/°C is typical of older PERC panels, and −0.38%/°C of polycrystalline. Closer to zero means less power lost on hot days.
How do you calculate solar panel power loss from heat?
Multiply the degrees of cell temperature above 25°C by the Pmax coefficient. A 400 W panel at −0.34%/°C with 65°C cells loses 40 × 0.34% = 13.6%, leaving about 346 W. Use cell temperature, not air temperature.
Is the temperature coefficient based on air temperature?
No, it is based on cell temperature. In sun, cells typically run 20°C or more above the air, as the NMOT figure on the data sheet shows. Roof-mounted panels run hotter than open-rack ones because less air flows behind them.
Do solar panels produce more power in cold weather?
Per unit of sunlight, yes. Below 25°C the negative coefficient works in reverse and output rises slightly. Winter days are shorter and the sun is lower, though, so total winter production is usually lower. Voltage also rises in cold, which installers plan for.
What is the difference between NOCT and NMOT?
Both describe how hot a panel’s cells run under a set field condition, and data sheets use either name. Check the conditions printed beside the figure. The sheets cited here use 800 W/m² of sun, 20°C air and a 1 m/s breeze.
Which solar panels handle heat best?
On the sheets we compared, heterojunction did best at −0.24%/°C, then back contact at −0.27% and TOPCon at −0.29%. Older PERC and poly panels lose more. Products vary within each type, so compare the exact data sheets you are quoted.
How hot do solar panels get?
Well above the air. Data-sheet NMOT values of 41–44°C show cells running about 20 to 25°C above 20°C air in mild conditions. NREL’s PVWatts V5 manual assumes roof mounts run hotter than open racks. On a hot summer day, cells in the 60s°C are plausible.
Why does heat reduce solar panel output?
Mainly because voltage drops as cells warm. On the REC Alpha Pure-RX sheet, voltage falls 0.24% per degree while current rises only 0.04%. Since power is voltage times current, power falls.
Does the temperature coefficient change over time?
Makers list one value for a new panel, and we have not found data-sheet guidance on how it changes with age. Aging is covered by the degradation rate in the warranty instead. Track both through your monitoring app.
Sources
- Canadian Solar, HiKu6 Mono PERC CS6R-MS data sheet V1.7C25_AU (April 2022), retrieved .
- REC Group, REC Alpha Pure-RX series data sheet (US), UL EN 6.2026 V7, retrieved .
- Maxeon Solar Technologies, Maxeon 7 435–445 W data sheet 551184 REV A (January 2024), retrieved .
- Canadian Solar, TOPHiKu6 CS6.2-54TM data sheet V1.2_EN (March 2025), retrieved .
- First Solar, Series 7 TR1 US module data sheet MPD-00640-07-US (April 2026), retrieved .
- Canadian Solar, KuMax CS3U-P (poly) data sheet V5.57_E2_NA (June 2018), retrieved .
- GAF Energy, Timberline Solar technical data sheet #837 SM210 (December 2021), retrieved .
- Dobos, NREL, PVWatts Version 5 Manual (NREL/TP-6A20-62641, September 2014), retrieved .
- UL Solutions, Building-Integrated Photovoltaic (BIPV) System Testing and Certification (UL 7103, UL 61730), retrieved .
- South Carolina Code Title 58 Ch. 40 (Act 62 of 2019, customer-generators), retrieved .
- Georgia Power, Behind-the-Meter Interconnection Summary for Residential Customers (rev. 15 Aug 2025), retrieved .
- Code of Virginia §56-594 (net energy metering), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Every coefficient and NMOT value was read from the named manufacturer data sheet PDF on the retrieval date.
Model defaults and roof-mount INOCT from the NREL PVWatts V5 manual. The worked example applies the standard linear formula to those coefficients; the Southeast cell-temperature range is labelled as an estimate derived from NMOT and INOCT.
Suggest a correction. We fix errors and say what changed.