How panels behave in the heat
A module is rated at a 25 °C cell temperature and almost never runs there. On a roof in summer the cell sits 20 to 50 °C above the air, and the temperature coefficient of Pmax says what that costs: the percentage of rated power lost per degree above 25.
It is the specification that separates two modules with the same efficiency on paper. Across 14,047 panels declaring one, the coefficient runs from -0.500 to -0.223 - a spread worth 11.1 percentage points of output on a hot afternoon, which is more than most price differences are worth.
What the distribution shows
8,068 panels hold -0.30 %/°C or better - the band worth having in a hot climate - and 1,693 are worse than -0.35, where a 40 °C rise costs more than 14% of rated output.
Heterojunction cells lead by construction: their lower operating voltage drift is exactly the property this coefficient measures. The axis starts at zero, so the gap reads as the fraction of a percent per degree that it is.
The coefficient is only half the story: NOCT is the cell temperature at 800 W/m² and 20 °C air, so a module with a low NOCT spends less time in the range where its coefficient hurts. The median is 45.0 °C across 13,011 panels declaring it, and the spread runs 39 to 70.
Best in the heat
At a cell temperature of 65 °C - an ordinary summer afternoon on a dark roof - the best module in the set keeps 91.1% of its rated power and the worst keeps 80.0%. On a 10 kWp array that is the difference between 9108 W and 8000 W of instantaneous output.
The coefficient is a rating, not a promise: it is measured at the cell, and how hot the cell actually gets depends on mounting, clearance and wind as much as on the module. Read it against NOCT rather than on its own.
By manufacturer
A brand that sells only heterojunction modules will lead this table on physics rather than on engineering, and a brand with a full range will sit mid-table while its best module beats everything above it.
Catalogues under 20 panels are marked in amber for the same reason: one product line moves a small average a long way.
Manufacturer insightsHow these numbers are built
Every figure comes from a manufacturer datasheet ingested into the panel database. Nothing is modelled, estimated, or supplied by a brand outside its own published specification.
Aggregates recompute as new panels arrive. A panel that does not declare a value is left out of that average rather than filled in, so every figure here is backed by modules that actually publish it.
No paid placements, and no figure here reads which shops pay us a commission. Each power rating counts as the module it is. Bifacial datasheets that quote one module at several assumed rear-side gains are counted once, on their front-side rating.