Cell technology and the temperature coefficient

In short: on their own datasheets, PERC modules in our catalogue state a median temperature coefficient of Pmax of -0.35 %/°C, TOPCon modules -0.29 %/°C and HJT modules -0.24 %/°C. The closer to zero, the less power a module loses for each degree above 25 °C, so in a hot climate the cell technology is a fair first filter. It's a filter, not a verdict: these are manufacturers' claims, and modules inside one technology still differ.

A panel's power rating is measured with its cells at 25 °C. On a summer roof they run much hotter, and the temperature coefficient of Pmax tells you how much power goes for each degree above that reference.

Modules are sold by cell technology, and each name arrives with a story about heat. This article checks one part of that story against something we can count: does the coefficient printed on the datasheet differ by technology, and by how much? It says nothing about field performance. This data can't.

How the modules were grouped

Every catalogue module that states both a cell technology and a Pmax coefficient went into a group. Five groups are large enough to compare: TOPCon, PERC, HJT, back contact (cells with all their contacts on the rear face), and modules whose datasheet says "N-type" without naming the structure.

I kept N-type apart from TOPCon. Many of those modules are probably TOPCon, but the label doesn't say so, and merging them would be a guess.

Across the catalogue, 13,923 modules state a coefficient. Of those, 1,014 have no cell technology recorded, so they fit no group. Another 118 sit in groups I set aside: CdTe, which here is one manufacturer's product line, and BIPV and flexible modules, which describe how a module is built, not what its cells are.

Modules versus datasheets

One datasheet usually covers a ladder of power ratings that share one coefficient. Each rating is its own catalogue row, so a ten-step ladder adds ten modules and one claim.

That's why the evidence table below gives two counts. Read the datasheet count as the honest sample size: if a few long ladders were pulling a median around, it would show there.

TOPCon, PERC and HJT on paper

PERC states the steepest heat loss, HJT the gentlest, TOPCon sits between them. Look at the median first, then at the two percentile columns, which bracket the middle eight in ten modules of each group. The values are negative, so the 10th percentile is the worse end.

Compare the 90th percentile of one row with the 10th percentile of the row below it. Where the two ranges don't touch, the technology label tells you more than any single datasheet inside the group.

TOPCon and the unspecified N-type group sit close together. That fits the guess that many "N-type" modules are TOPCon under another name. The label still can't confirm it.

HJT cells put thin layers of amorphous silicon on both faces of a crystalline wafer. The photo below shows one on a measurement stage.

How much evidence each group rests on

Before you trust a gap, check what stands behind it. The column to watch is datasheets, not modules.

HJT and back contact rest on far fewer datasheets than PERC and TOPCon, so a handful of manufacturers can move their medians more. Each still draws on many separate files from several makers.

Three distributions, one module from each

The three charts share one horizontal axis, so you can compare their shapes directly. Under each sits one real module from that group.

I picked them by a fixed rule, not by brand: a module with all four scorecard figures whose coefficient sat at its group's median when this was written. Nobody paid for the spot. The verdict on each card compares the module with the whole catalogue, so a perfectly middling PERC module can still read as weak overall.

PERC first, with its median at -0.35 %/°C over 4,283 modules.

TOPCon, with a median of -0.29 %/°C over 5,721 modules.

HJT, with a median of -0.24 %/°C over 697 modules.

Why the groups might differ

Heat costs a solar cell voltage, and voltage is where most of the loss comes from. As the cell warms, its open-circuit voltage drops noticeably while its current rises only a little, so their product, the power, falls.

PVEducation adds one documented thread that may tie this to technology: cells with a higher voltage are less affected by temperature. Whether that explains the table, this data can't show. I compared coefficients, not cell voltages, and a datasheet coefficient bundles every effect into one number.

The thread runs through the cell structure. In the figure below, turn up the losses at the rear contact and watch voltage and heat loss move against each other.

So read the table as what manufacturers state, grouped by the technology they name. It's an observation, not a proof of why.

What a better coefficient is worth on a hot roof

The gap only matters while the cells are hot. Here is one such hour, with two round coefficients picked for easy arithmetic; hold them against the medians above yourself.

Take a 430 W module whose cells reach 60 °C, which is 35 degrees above the rating condition (60 - 25). The last column is what you get.

The module closer to zero delivers about 9 W more, roughly 2 % of its rating.

The figure below starts both modules at the catalogue medians for TOPCon and PERC. Move the air temperature and watch where the gap opens.

On a cool morning both modules sit near their rating and the difference shrinks towards zero. So a 2 % gap in the hottest hour is not a 2 % gap in yearly energy; how much it adds up to depends on your climate and how well the array is ventilated. How to choose panels for a hot climate sets the coefficient next to the NOCT for one hot hour.

What this means when you buy

In a hot climate, start with the technology. Picking from a group with a median closer to zero moves the odds your way before you open a single datasheet.

Then stop trusting the group. Two modules sold under the same name can differ, so once you're down to two candidates, compare their own datasheets, not the median.

And keep the coefficient in proportion. Price per watt, efficiency and degradation also shape what a system earns. In a mild climate I wouldn't pay much extra for a better coefficient alone. To see whether a few percent of hot-hour yield changes your payback, put the numbers into the solar payback calculator.

If you want the modules at the far end of the spread, the catalogue picks them by the rule in the caption:

Top 3 by the smallest Pmax temperature coefficient, one module per datasheet, among 12,632 modules of 300 W or more with a stated cell technology and a stated Pmax temperature coefficient, as of the last catalogue sync.

Caveats

Every number here is a manufacturer's claim, measured on sample modules under controlled conditions. We haven't tested any module ourselves, and two modules with the same printed coefficient can behave differently on a roof.

The coefficient is a linear model around standard test conditions: 1000 W/m², the AM1.5 spectrum and 25 °C. Real arrays see changing irradiance, wind and mounting, so the worked example isolates temperature rather than predicting output.

The technology label is whatever the datasheet states. It can be broad ("N-type") or missing, and modules without one are left out rather than guessed. If the unlabelled modules differ systematically from the labelled ones, the medians wouldn't show it.

Finally, the catalogue counts what is listed, not what is sold. Many datasheets don't mean many installations.