Understanding temperature derate
Why heat costs power instead of adding it
On a 95°F day with a standard monocrystalline panel mounted close to a dark roof, cell temperature can reach around 66°C, cutting peak power output by roughly 15% to 16% compared to the panel's rated capacity at 25°C. Solar panels are rated at a standard 25°C cell temperature, and their voltage output actually drops as silicon cells heat up, even though more sunlight usually comes with warmer weather. This is called the temperature coefficient, and it's a spec every panel has, typically listed on the datasheet. A DOE federal PV performance report lists temperature-driven derating among the standard predictable losses built into most performance models.
Panel technology changes the sensitivity
Monocrystalline panels typically have a temperature coefficient around negative 0.35% per degree Celsius, polycrystalline panels run closer to negative 0.40%, and thin-film panels often perform better in heat at around negative 0.25%. This is one reason thin-film technology sometimes gets chosen for very hot climates despite lower baseline efficiency. NLR's PVWatts documentation lists standard crystalline silicon modules at negative 0.37% per degree Celsius and premium modules at negative 0.35%, figures consistent with the module-type spread described here.
Mounting airflow matters as much as the weather
Panels mounted flush against a roof with little airflow underneath can run 25°C to 30°C hotter than ambient air, while an open rack mount with airflow on all sides often stays closer to 20°C above ambient. This gap alone can shift your derate estimate by several percentage points, independent of your local climate. If mounting choice is still an open question for your project, the ground mount vs roof mount space calculator can help weigh this airflow benefit against the space and cost tradeoffs of each option.
What this number does and doesn't tell you
This calculator shows peak instantaneous power loss under summer heat conditions, not a full annual energy loss figure. The real annual impact is smaller, since this derate only applies during the hottest hours of the hottest days, not the whole year. For a broader annual view, check the hot climate efficiency loss calculator or the desert heat impact calculator if you're in an especially hot region.
What to do with this number next
Compare against the cold climate efficiency gain calculator to see the flip side of the same physics, or check the real world vs nameplate gap calculator to see how temperature fits alongside other real-world losses. Heat hits the inverter as well as the panels, and an inverter running hot converts a smaller share of the DC it receives. The inverter efficiency loss calculator puts a number on that conversion loss.