A panel is rated at 25 degrees. It works at 53.
The wattage on a solar panel's datasheet is measured at Standard Test Conditions: a cell temperature of 25 degrees Celsius. A working panel on a real roof runs far hotter, because it sits in the sun absorbing the very irradiance it converts. Across the 40 markets in Solar Analytica's yield baseline, the modelled annual operating cell temperature averages 53.1 degrees, and reaches 65.8 degrees in the hottest sites (Bamako in the Sahel and Jodhpur in Rajasthan). Every degree above 25 shaves output: mainstream mono-silicon (PERC-class) datasheets state a power temperature coefficient of roughly a third of a per cent per degree, and heterojunction (HJT) cells roughly a quarter of a per cent, a gap that matters most in hot markets.
53.1°C
The average modelled operating cell temperature across the 40 markets, for panels whose wattage is rated at 25°C.
Costed across a full year of weather, that gap between the rating plate and the roof is the static heat tax. The average market in the baseline loses 9.8 per cent of nameplate output to cell temperature with PERC-class modules, and 6.7 per cent with HJT; the hottest markets lose more than a seventh:
| Market | Cell temp °C | Heat loss, PERC | Heat loss, HJT | Days above 35°C/yr |
|---|---|---|---|---|
| Bamako (Sahel), Mali | 65.8 | 14.3% | 9.8% | 165 |
| Jodhpur (Rajasthan), India | 65.8 | 14.3% | 9.8% | 180 |
| São Paulo, Brazil | 56.8 | 11.1% | 7.6% | 2 |
| Seville, Spain | 56.6 | 11.1% | 7.6% | 68 |
| Perth, Australia | 53.3 | 9.9% | 6.8% | 2 |
| Berlin, Germany | 45.4 | 7.1% | 4.9% | 1 |
| Helsinki, Finland | 39.2 | 5.0% | 3.4% | 0 |
| Reykjavik, Iceland | 37.7 | 4.4% | 3.0% | 0 |
Solar Analytica yield model: modelled annual operating cell temperature and the resulting temperature-coefficient loss against nameplate, from NASA MERRA-2 (1994-2024) temperatures and NASA CERES plane-of-array irradiance. Days above 35°C are 30-year averages, rounded. Set averages across all 40 markets: 53.1°C, 9.8% (PERC), 6.7% (HJT).
The record: two decades of measured warming
The static tax is priced against today's climate. The second question is how that climate has moved. NASA's GISTEMP v4 global surface record puts the global anomaly at +0.53°C in 2004 and +1.28°C in 2024, the warmest year in the instrumental record, against the 1951-1980 baseline. Averaged in five-year windows to strip single-year noise, the global anomaly rose from +0.61°C (2004-2008) to +1.08°C (2021-2025).
The global anomaly is one number; the 40 local trends spread widely around it. Fitting a linear trend to each market's own 30-year MERRA-2 temperature series (1994-2024), the 40 baseline markets warm at +0.39°C per decade on average, but the spread is wide: Berlin warms at +0.95°C per decade, Seoul at +0.80, São Paulo at +0.76, while four markets in the set (Cape Town, Bamako, Reykjavik and Jodhpur) show a flat-to-negative local trend within this specific 30-year window. A local 30-year fit is an observation, and no guarantee of the next 25 years, but it is a far better planning input than a global average applied everywhere.
The warming penalty, market by market
Projecting each market's own trend forward across a 25-year system life and passing it through the module's temperature coefficient gives the warming penalty: the additional heat loss a system faces in its final year, on top of the static tax it already pays.
| Market | Local trend °C/decade | Added loss by year 25, PERC | Added loss, HJT |
|---|---|---|---|
| Berlin, Germany | +0.95 | 0.83% | 0.57% |
| Seoul, South Korea | +0.80 | 0.70% | 0.48% |
| São Paulo, Brazil | +0.76 | 0.67% | 0.46% |
| Albuquerque, USA | +0.74 | 0.65% | 0.44% |
| Madrid, Spain | +0.72 | 0.63% | 0.43% |
| Perth, Australia | +0.38 | 0.33% | 0.23% |
| Jodhpur (Rajasthan), India | -0.17 | -0.15% | -0.10% |
| Cape Town, South Africa | -0.38 | -0.33% | -0.23% |
Solar Analytica yield model: each market's 1994-2024 MERRA-2 linear temperature trend, projected 25 years and converted to output loss through the module temperature coefficient. Set averages: +0.34% (PERC), +0.23% (HJT). Negative values reflect a negative local trend in the observation window, not a forecast of cooling.
The average warming penalty is about a third of a per cent by year 25, roughly one thirtieth of the static heat tax the same systems already pay today. Sharper still is the geography: the largest penalties land in temperate mid-latitude cities, not in the deserts. The hottest markets pay the largest static tax and, in this observation window, some of the smallest warming drifts.
Two questions inside one
"Will warming hurt my solar system?" is really two questions, and the record answers them differently. The cost of heat today runs around a tenth of nameplate in the average market, more than a seventh in the hottest, every year from day one. The addition from warming is small on a system lifetime: a third of a per cent on average by year 25, under one per cent even in the fastest-warming market in the set.
Converted to money through Solar Analytica's retail-tariff and yield join (tariff provenance in the electricity-prices report), the contrast sharpens. In Berlin, the fastest-warming market in the baseline, the warming penalty at year 25 costs about US$4 per kilowatt of panels per year at 2025 retail prices. In Perth it is about US$1.20. Choosing an HJT module over a PERC-class module in the same two markets is worth about US$11 and US$13 per kilowatt per year respectively, three to ten times the warming drift, because it applies to the full static tax rather than the marginal creep. Annual module degradation, warranted at around 0.4 per cent per year in mainstream module warranties, also outweighs the warming drift roughly tenfold over a system's life.
The order of operations for a buyer follows directly. Reject any production estimate quoted at the nameplate rating and insist on a yield figure modelled at operating cell temperature for the location, because the 9.8 per cent average gap is the largest single correction in this report. In hot markets the temperature coefficient is the specification to shop on, the main heat lever on the datasheet. And the warming drift belongs in a 25-year cash-flow model as a rounding term.
Thirty years of temperature, forty markets
The figures here come from Solar Analytica's location yield baseline: 40 global markets, each carrying a 30-year NASA MERRA-2 temperature series (1994-2024), NASA CERES satellite irradiance, a cell-temperature model of the plane-of-array environment, and datasheet-class temperature coefficients for PERC and HJT module families. The warming penalty is that market's linear 30-year trend, projected 25 years and passed through the coefficient. A local linear fit over one 30-year window is an observation with real variance, and the four negative-trend markets illustrate exactly that; they read as flat rather than as cooling forecasts. The model holds irradiance constant while varying temperature; brightening and dimming of the resource itself is a separate variable with its own 20-year record, covered in part 02 of this series. And soiling, smoke and extreme events sit outside this model entirely; they are parts 05, 06 and 08. Every input series is public and named here or in the linked tariff report, so the numbers can be recomputed.