Better Temp Coefficient or Cooler City? What Moves Hot-Climate Yield

Method note

Modeled 8 kW south-facing fixed arrays at Shiraz (29.6106°N, 52.5245°E) and Isfahan (32.6539°N, 51.6660°E). Weather is JRC PVGIS TMY (API v5_2), run 2026-07-26. Hourly AC uses pvlib 0.11.2 (PVWatts DC + Faiman cell temperature + PVWatts inverter), tilt 29°, azimuth 180°, 14% system loss. Module Pmax temperature coefficients come from Canadian Solar datasheets: HiKu6 CS6W-MS mono PERC at −0.34%/°C and TOPHiKu6 CS6.1-54TM-H n-type TOPCon at −0.29%/°C. Nameplate kW is held constant so only γPmax (and city weather) change. Background: temperature coefficient and peak sun hours.

Direct answer

On Shiraz, swapping PERC (−0.34%/°C) for TOPCon (−0.29%/°C) lifts annual AC from 13,665 kWh to 13,811 kWh (+146 kWh, about +1.1%). Moving the same PERC array to Isfahan instead yields 13,943 kWh (+278 kWh, about +2.0% vs Shiraz PERC). So the datasheet coefficient step is real and worth checking, but a cooler nearby climate with similar plane-of-array irradiance moved yearly energy more than that 0.05 percentage-point coefficient gap in this pair. Peak sun hours alone would have pointed the wrong way: Shiraz’s average daily GHI/1000 is 5.68 vs Isfahan 5.59, yet Isfahan still wins on AC.

Hypothesis

A datasheet step from −0.34%/°C to −0.29%/°C is a meaningful hot-climate lever, but a nearby cooler city with similar plane-of-array irradiance can change annual AC by as much as, or more than, that coefficient step alone.

Inputs

Climate and irradiance box

Shiraz has slightly more horizontal irradiance. Isfahan has slightly more plane-of-array energy at this shared tilt and runs cooler cells. That combination explains why “more sun on the map” is not the same as “more AC on the meter.”

Scenario results (annual AC)

Figure: annual AC for the four modeled scenarios (pvlib hourly, 2026-07-26 run).

Where the coefficient shows up in the year

The TOPCon advantage on Shiraz is small in winter and larger in hot months (July about +23 kWh, August about +21 kWh). That matches the physics: cell temperature rises with irradiance, so γPmax bites hardest when roofs are hottest.

Figure: monthly AC shape for Shiraz PERC, Shiraz TOPCon, and Isfahan PERC.

What this means

Hot-climate buyers often chase peak sun hours or STC watts. This run separates two levers that proposals mix together: (1) the module’s Pmax temperature coefficient, and (2) the site’s thermal and plane-of-array climate. A 0.05 percentage-point better coefficient recovered about 1% annual energy on Shiraz. The Isfahan move recovered about 2% with the same PERC datasheet, even though its GHI-based PSH proxy was slightly lower. Principle: compare modules on the same weather file, and compare sites with production models, not GHI rank alone. See also what affects solar production and n-type cells.

Uncertainty and limits

- TMY is a typical year, not a forecast. Interannual weather can move annual yield by several percent. - Faiman cell temperature is a model, not a rooftop thermocouple. Flush mounts with poor rear airflow run hotter than open racks. - Both cities use the same 29° tilt for a fair coefficient test; a site-optimal tilt per city would change POA slightly. - Only γPmax changes between PERC and TOPCon here. Real modules also differ in NMOT, efficiency, bifaciality, and degradation. - No soiling, shade, or clipping sweep in this study.

What to do with this

1. Read γPmax on every shortlisted datasheet (and keep the PDF). 2. Ask for an hourly production run on your coordinate, not only a PSH table. 3. When comparing two cities or two roofs, hold module assumptions fixed, then swap modules on one weather file. 4. Treat a ~1% coefficient lift as real but easy to lose to shade, soiling, or a hotter microclimate.

Closing

For this Shiraz hourly climate, moving from −0.34%/°C to −0.29%/°C adds about 146 kWh/year on 8 kW. Relocating the same PERC assumptions to cooler Isfahan adds about 278 kWh/year, even with a slightly lower GHI-based PSH proxy. Coefficient shopping helps; climate physics still sets the stage.