Tấm pin quang điệnPhotovoltaic panels · BàiLesson 4/7
Temperature and Energy Yield
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Solar Knowledge HubNhiệt độ và sản lượngTemperature and yield
Một hiểu lầm rất phổ biến là trời càng nắng gắt thì tấm pin càng ra nhiều điện. Thực tế ngược lại: nắng nóng làm tấm pin nóng lên và mất công suất.A very common misconception is that the hotter the sun the more power the panel produces. In reality the opposite is true, heat causes panels to lose power.
Môi trường 40°C → tấm nóng 60–70°C → cao hơn chuẩn ~40°C. Với hệ số −0.34%/°C: 0.34 × 40 = 13.6% mất công suất. Tấm 600W chỉ còn khoảng 518W.Ambient 40°C → panel at 60–70°C → ~40°C above reference. At −0.34%/°C: 0.34 × 40 = 13.6% power loss. A 600W panel produces only about 518W.
- Chọn tấm hệ số nhiệt tốt: TOPCon, HJT hoặc back-contact loại N: hợp khí hậu nóng hơn PERC cũ.Choose a good temperature coefficient: TOPCon, HJT, or N-type back-contact: better for hot climates than old PERC.
- Lắp thoáng: chừa khe hở ~10–20 cm giữa tấm và mái để không khí lưu thông làm mát mặt sau; tránh áp sát mái tôn hấp nhiệt.Mount with an air gap ~10–20 cm between panel and roof for rear ventilation; never flush on a heat-absorbing metal roof.
- Theo dõi nhiệt độ qua hệ giám sát để phát hiện sớm chỗ lắp bí nóng bất thường.Monitor panel temperature via the monitoring system to catch unusually hot spots early.
NMOT đáng đọc hơn STC khi ước lượng sản lượng thực. STC đo ở 25°C tế bào, điều kiện hiếm gặp ngoài trời nắng. NMOT đo ở điều kiện gần thực tế hơn nên cho con số sát với những gì khách thấy trên app. Khi tính sản lượng năm, dùng con số có tính tới nhiệt thay vì lấy thẳng công suất STC.NMOT is more useful than STC for estimating real yield. STC measures at 25°C cell temperature, rarely encountered in real sunlight. NMOT is closer to field conditions and matches what customers see in their monitoring app. Use a temperature-adjusted figure for annual yield calculations rather than raw STC power.
Khi khách thắc mắc vì sao trưa nắng gắt mà sản lượng không cao như kỳ vọng: "Tấm nóng mất công suất: đó là vật lý chung của mọi tấm, không phải lỗi sản phẩm." Điểm bán tích cực: tấm loại N của hệ chịu nóng tốt hơn tấm đời cũ và cách lắp thoáng giúp giữ sản lượng. Đừng hứa nắng càng to điện càng nhiều.When customers ask why midday output is below expectations: "Hot panels lose power: that's physics, not a product defect." Positive selling point: the N-type panels in the system handle heat better than older-generation panels, and proper ventilation helps maintain yield. Never promise that more sun always means more power.
A very common misconception is that the more intense the sunshine, the more electricity a panel produces. In reality the opposite is true: intense heat warms the panel up and causes it to lose power. This document explains why, quantifies the loss, and draws out installation practices that minimise the damage.
For: sales staff answering customer questions about summer-season yield, technicians selecting panels and planning layouts, newcomers who need to understand the temperature coefficient.
Quick summary
More sun does not equal more electricity. What generates electricity is irradiance; high temperature, on the other hand, causes the panel to lose power, so the two factors pull against each other at midday in summer.
Every degree Celsius above the standard reference of 25°C reduces power by roughly 0.3 to 0.4%, depending on panel technology. At midday when ambient temperature is 40°C, a panel exposed to full sun can reach 60 to 70°C, resulting in a power loss of around 13 to 14%.
The ways to limit this loss are to choose panels with a good temperature coefficient and to mount them with enough ventilation for heat to dissipate, leaving an air gap from the roof rather than pressing the panels flush against a metal sheet roof.
Why intense heat reduces power output
Two concepts that are often lumped together need to be separated. Irradiance is the amount of light striking the panel surface, and this is what actually generates current: the more light, the more current. Cell temperature is how hot the panel gets, and it works in the opposite direction, reducing voltage and increasing internal losses within the cell.
At midday in summer both factors are present simultaneously but pulling in opposite directions. Strong sunlight adds a little extra current, yet high temperature strips away more power than that increment provides, so the net result is lower yield. This is why a cool morning with good sunshine typically delivers higher output per kilowatt of installed capacity than a scorching midday.
Quantifying the loss
The loss is measured by the temperature coefficient, the percentage of power lost per degree Celsius above the 25°C reference. Most panels today have a coefficient of around −0.3 to −0.4% per °C, with N-type panels performing better than older P-type panels.
A concrete example: when ambient temperature is around 40°C, a panel in full sun can heat up to 60–70°C, which is roughly 40°C above the reference. At a coefficient of −0.34%/°C, power falls by 0.34 × 40 ≈ 13.6%. A panel rated at 600W would then deliver only about 518W. This is not a defect. It is the normal physics of photovoltaic panels, and it explains why real-world output is always lower than the nameplate rating.
| Panel condition | Yield |
|---|---|
| Cool cells, ~25°C | Highest |
| Hot cells, 60–70°C | Noticeably lower |
This is also why the NMOT specification is worth reading alongside STC when estimating real-world yield. STC is measured at a cell temperature of 25°C, a condition rarely encountered outdoors in full sun. NMOT is measured under conditions closer to reality, so it gives figures that better match what customers see in their monitoring app. When calculating annual yield, it is better to use a figure that accounts for temperature rather than taking STC power at face value.
How to reduce heat-related losses
There are two levers. The first is to choose panels with a good temperature coefficient, that is, a smaller negative number. N-type panels such as TOPCon, HJT, and back-contact handle heat better than older PERC panels, as compared in document 02-02, making them well suited to Vietnam’s hot climate.
The second is to mount panels with adequate ventilation so heat can dissipate. Leave an air gap of around 10 to 20 cm between the panel and the roof surface so that air can circulate and cool the rear face; avoid mounting flush against a metal sheet roof that absorbs heat and bakes the panel from below. Monitoring cell temperature through a system monitoring platform also helps identify mounting locations with abnormally high heat build-up at an early stage.
When a customer asks why yield is lower than expected during intense midday sunshine, the honest answer is that the panel gets hot and loses power, and that is the universal physics of every panel, not a product defect. The positive selling point is that the N-type panels in the system handle heat better than older-generation panels, and an air-gap installation helps preserve yield. Never promise that more sunshine means more electricity, because reality will erode customer trust.
Safety considerations
For outdoor PV arrays, beyond the thermal issue there are also voltage and arc-flash risks to consider. DC strings operating at 600 to 1000 V are extremely dangerous, so systems require rapid shutdown capability to de-energise on fault, as well as arc-fault circuit interruption (AFCI) devices. These protection layers are covered in detail in Chapter 01.
The principle that high temperature reduces power output is per Sinovoltaics and The Green Watt. Panel temperatures reaching 60–70°C under 40°C ambient are per Solar Tech Online and EcoFlow. The temperature coefficient range of approximately −0.3 to −0.4%/°C is per SurgePV and WINAICO. The calculation of 40°C × 0.34% = 13.6% loss and 600W → ~518W has been arithmetically verified and matches the source material. Rapid shutdown per NEC 690.12 and AFCI per NEC 690.11; for Vietnam, cross-reference with TCVN standards and EVN requirements.