Two 400 W panels, one hot roof, 14 watts apart: what temperature coefficient predicts that efficiency alone doesn't.
Two panels, same 400 W rating, same roof. On a hot afternoon, one delivers 14 watts more than the other. The difference is a single line most buyers skip on the datasheet, and this is how to read it before you buy.
Temperature coefficient of Pmax is the percentage a panel's power falls for every degree its cells run above the 25°C reference.
It is always negative for crystalline silicon: power falls as cells heat up. A module rated at −0.35%/°C loses 0.35% of its nameplate output for every degree above 25°C. The figure is defined by IEC 60891 and measured under IEC 61215's MQT 04 procedure: a module sits in a temperature-controlled chamber under constant 1,000 W/m² illumination, full current-voltage curves are recorded across a minimum 30°C span, and the coefficient is the slope of a linear regression of maximum power against temperature. Inter-laboratory testing puts the measurement uncertainty at roughly ±0.02%/°C, a threshold worth keeping in mind when two products' datasheets differ by less than that.
Every datasheet's temperature characteristics table carries three coefficients: Pmax (negative, the one that drives yield), Voc (negative, typically −0.24% to −0.32%/°C, the figure installers use for cold-weather string-voltage limits), and Isc (slightly positive, typically +0.04% to +0.06%/°C, and immaterial to the power outcome). Pmax is the one this piece is about.
Why the sticker number overstates the roofSTC's 25°C is a cell temperature, not an air temperature, and real cells run well past it.
STC was built as a comparability standard, not a performance predictor. The standard's intermediate reference, NOCT (Nominal Operating Cell Temperature, defined at 800 W/m², 20°C ambient, 1 m/s wind), sits between 42°C and 48°C for most crystalline silicon modules. In practice, field cell temperatures span roughly 35°C to 75°C depending on climate, mounting and wind: a utility-scale PERC array in Arizona reported around 55°C module temperature at 40°C ambient in summer, and rooftop installations in Rajkot and Gujarat, India, regularly exceed 70 to 75°C cell temperature in summer.
Run that through the governing formula, actual power equals rated power times [1 + (TC ÷ 100) × (cell temperature − 25)], and the gap becomes concrete. A 400 W module at −0.35%/°C, at a cell temperature of 65°C (a 40°C rise above STC): 400 × [1 + (−0.0035 × 40)] = 400 × 0.86 = 344 W. The same 400 W nameplate rating at −0.26%/°C, same 65°C: 400 × [1 + (−0.0026 × 40)] = 400 × 0.896 = 358 W. Fourteen watts apart, same STC rating, before any other variable is considered.
The physics behind the gapHeat suppresses voltage before it touches current, and some cell architectures suppress that suppression better than others.
In a silicon photovoltaic cell, power falls with temperature primarily because rising heat increases the intrinsic carrier density, which increases the dark saturation current density, which suppresses open-circuit voltage. Since power is the product of current and voltage, a higher retained Voc at operating temperature is most of the power-loss story: short-circuit current actually rises slightly with heat, but that effect is minor next to the Voc-driven loss.
Heterojunction (HJT) cells achieve a lower dark saturation current through their defining structural feature, a layer of hydrogenated amorphous silicon that passivates the crystalline silicon surface and suppresses recombination at the interface. That passivation is what lets HJT hold a higher Voc under heat than a conventional diffused junction. Published device-physics data traces the progression across cell generations: standard homojunction silicon runs about −0.45%/°C, passivated-contact designs (the PERC-class improvement) reach about −0.35%/°C, and HJT reaches about −0.23%/°C under test conditions. At 70°C cell temperature, that progression translates to a 5.4% to 15.8% relative difference in temperature-related efficiency loss between homojunction and HJT designs, depending on the exact device parameters.
What independent testing showsPVEL's 2025 Scorecard puts HJT's average coefficient at −0.25%/°C, TOPCon's at −0.29%/°C, PERC's at −0.32%/°C.
The Kiwa PVEL PV Module Reliability Scorecard is the cleanest independent source for TC Pmax by technology class: it averages results across dozens of manufacturers submitted for testing each year, rather than relying on any single vendor's claim. The gap between classes has narrowed slightly year on year (HJT ran −0.26%/°C in 2023 and 2024; PERC ran −0.33%/°C in 2023), but the ranking hasn't moved. PVEL's VP of module testing, Tristan Erion-Lorico, put it directly in 2023: "With TOPCon and HJT we're definitely seeing higher performance from those technologies for PAN testing... A lot of that comes from the temperature coefficients of these modules just being, frankly, much better than what we saw with PERC."
Named products show the same ordering, with real spread inside each class:
| Model | Technology | TC Pmax | NOCT / NMOT |
|---|---|---|---|
| REC Alpha Pure-RX | HJT | −0.24%/°C | 44 ± 2°C |
| Risen Hyper-ion | HJT | −0.24%/°C | 43 ± 2°C |
| REC Alpha Pure-R | HJT | −0.26%/°C | 44 ± 2°C |
| Canadian Solar HiHero | HJT | −0.26%/°C | 41 ± 3°C |
| Jinko Tiger Neo (72HL4-BDV) | TOPCon | −0.30%/°C | 45 ± 2°C |
| LONGi Hi-MO 9 | Back-contact (HPBC) | −0.26%/°C | not disclosed |
| Maxeon 7 | Back-contact (IBC) | −0.27%/°C | 41.5°C |
Manufacturer datasheet values, current to early 2026, not independently re-verified for this table. Sources: REC, Risen, Canadian Solar, Jinko, LONGi and Maxeon product datasheets.
Chart data
| Series | PVEL 2025 Scorecard average | Low | Datasheet range across named products base | High |
|---|---|---|---|---|
| HJT | 0.25%/°C | 0.24%/°C | 0.25%/°C | 0.26%/°C |
| TOPCon | 0.29%/°C | 0.28%/°C | 0.3%/°C | 0.32%/°C |
| PERC | 0.32%/°C | 0.32%/°C | 0.355%/°C | 0.39%/°C |
The measurement's ±0.02%/°C inter-laboratory noise floor means a difference smaller than that between two specific products isn't, by itself, a reliable basis for choosing between them.
What that gap costs in the fieldField studies in hot climates report annual yield gaps in the same direction as the instantaneous numbers, four to nine per cent.
The formula above produces a calculated table, not a field measurement:
Published field studies in hot-climate installations track in the same direction. A 2022 study in Solar Energy monitored 24 modules per technology for a full year in Hainan, China: HJT bifacial exceeded PERC bifacial by 8.79% annual energy yield, and TOPCon exceeded PERC by 5.55%. Risen Energy's multi-region field data reported HJT modules generating 4.37% to 6.54% more power than PERC and 1.25% to 3.33% more than TOPCon globally, with the largest gains, over 6% against PERC and over 3% against TOPCon, in the Middle East, Australia and the southern United States. LONGi's global field programme for its Hi-MO 9 back-contact module reported a 1.54% generation gain over TOPCon at a Sanya, Hainan test site (with a 1.21°C lower backsheet operating temperature) and 1.87% at a Zhejiang site. A Risen Hyper-ion Pro module, SGS-tested in Saudi Arabia, generated 4.09% more than TOPCon annually and 6.04% more in August, the peak summer month.
The IEA PVPS Task 13-32:2025 report, using the standardised IEC 61853 Climate-Specific Energy Rating methodology across six climate profiles, is the more conservative reference point: it puts annual kWh/kWp differences among mainstream silicon technologies at roughly ±2.5% or lower once averaged across a full climate year. Both figures are correct in their own frame. The CSER number is the right reference for comparing technologies across a full year in an arbitrary climate. The field-study percentages are the right reference for what happens when a system spends real time above 55°C, which is exactly the condition the hot-climate studies above were run in.
Where the datasheet number runs outPVEL's independent PAN testing checks 23 points across irradiance and temperature. A datasheet gives you one.
PVEL's PAN Performance test, run under IEC 61853-1, measures three identical modules across 23 operating points spanning 100 to 1,100 W/m² irradiance and 15°C to 75°C, then produces a PAN file for use in PVsyst energy-yield modelling, checked against reference simulations for a temperate site (Boston) and a desert site (Las Vegas). Kiwa PI Berlin's own measurements found that manufacturer-supplied PAN files, built from datasheet values rather than independent testing, systematically overstated low-light specific energy yield by 1.1% to 2.9% compared to independently measured files. Where a product has independent PAN data, that supersedes the datasheet TC figure as a yield predictor.
One nuance worth flagging rather than smoothing over: PVEL's Incidence Angle Modifier data shows HJT modules with a 0.2% lower energy-yield impact than TOPCon and PERC on a modelled single-axis tracker site in Las Vegas, likely from blue-light absorption differences in the amorphous silicon layer. It is a minor negative for HJT, and a reminder that no single coefficient describes the whole of a module's field behaviour.
What this means for the panels on this benchmarkTemperature coefficient is a scored input on this benchmark, not a marketing footnote.
Every panel on review.solar carries a sourced temperature coefficient figure inside the Efficiency & Yield criterion, alongside NOCT, independent energy yield, low-light/IAM response and bifaciality: what a panel delivers in real conditions, not just at STC. The full definition sits in the glossary.
Three checks are worth making on any datasheet before the technology label does the deciding: where the figure sits inside its own technology class's range, not just which class it belongs to; the NOCT or NMOT value, since a lower figure reaches lower peak cell temperatures and amplifies whatever TC advantage a module has; and whether independent PVEL PAN data exists for the product, since it supersedes the datasheet figure entirely when available.
The financial size of this signal depends on climate. It is largest in hot, high-irradiance regions: the desert Southwest United States, the Middle East, Australia, India, southern China, where cell temperatures regularly exceed 55 to 60°C for extended periods and the field studies above show 4% to 9% annual advantages holding up. Huasun's own lifecycle analysis for a Middle East commercial rooftop modelled 24,981 kWh more energy for HJT than PERC per module over 25 years, and a lower resulting LCOE: a manufacturer-originated figure, indicative rather than independently verified, but directionally consistent with the independent field studies above. In temperate and cold climates the advantage shrinks: the Fraunhofer/IEA PVPS report notes that a higher TC can even produce a slightly higher yield where modules spend real time below 25°C, and the standardised CSER spread of ±2.5% or lower reflects that moderation. In those climates, degradation rate, bifaciality and price per watt more often decide the comparison.
Full panel scoring criteria: review.solar/methodology.
Sources17 references
- IEC 60891:2021: Photovoltaic devices: procedures for temperature and irradiance corrections
- PVMET Wiki: Measurement of the temperature coefficient (IEC 61215 MQT 04)
- Tongwei: Temperature coefficient inter-laboratory comparison, citing NREL test data
- NREL: NOCT measurement report (fy10osti/49505)
- PVEducation.org: Module measurement and the NOCT cell-temperature model
- OSTI / US DOE: Silicon heterojunction solar cells, device physics review
- Kiwa PVEL: 2023 PV Module Reliability Scorecard executive summary
- Kiwa PVEL: 2024 PV Module Reliability Scorecard
- Kiwa PVEL: 2025 Scorecard Insights
- PV Tech: PVEL's Tristan Erion-Lorico on 2023's solar module performance trends
- Solar Energy (ScienceDirect, 2022): Field comparison of HJT, TOPCon and PERC yield in Hainan, China
- pv magazine: Risen Energy global power generation gains, comparison map and technical analysis
- pv magazine: LONGi publishes global field data for Hi-MO 9 back-contact modules
- PV Tech: Risen's mass-produced HJT modules, Saudi Arabia SGS field test
- IEA PVPS Task 13-32:2025: Climate optimisation report (Fraunhofer CSP, SUPSI, Sapienza University)
- Kiwa: PAN Performance and IAM testing, PI Berlin comparison data
- TaiyangNews: Huasun Energy LCOE analysis, commercial/industrial rooftop HJT vs. TOPCon vs. PERC
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