Hot Weather: Heat Derating for Fixed vs Flexible vs Blanket Panels

Modified on Wed, 5 Nov at 12:42 AM

Solar panels love sunshine — but not heat. While clear Australian skies deliver high irradiance, extreme temperatures reduce panel voltage and total wattage. This article explains how heat derating works, what to expect from different panel types (fixed, flexible and blanket), and how to optimise performance during summer conditions across Australia.

Why Solar Panels Lose Power in the Heat

Every photovoltaic (PV) cell has a temperature coefficient — a small percentage drop in output per °C rise above 25 °C (the “Standard Test Condition” or STC).

Typical coefficient: -0.35 % to -0.45 % / °C
Example: If a 400 W panel has -0.38 % / °C and cell temperature climbs to 65 °C, output falls by about 15 % (40 °C × 0.38 % = 15.2 % loss) → 400 W × 0.848 ≈ 340 W.

This is called heat derating — perfectly normal, not a fault.

Real-World Temperatures in Australia

Under peak sun, cell temperature (NOCT) is usually 20–25 °C hotter than ambient.

Ambient (°C)	Cell Temp (°C)	Approx. Power Loss
25 °C	45–50 °C	–8 %
35 °C	55–60 °C	–12 %
45 °C	65–70 °C	–15–18 %
50 °C +	70–80 °C	–20–25 %

Heat Derating by Panel Type

Panel Type	Construction	Cooling	Typical Loss on 40 °C Day	Notes
Fixed (Glass-Framed)	Rigid aluminium frame + tempered glass	Air gap underneath aids convection	-12 – 15 %	Performs best in sustained heat; ideal for roofs or ground-mounts.
Flexible Panels	ETFE/PET laminate bonded to surface	Limited airflow → trapped heat	-18 – 22 %	Expect 5–7 % extra loss on metal roofs; keep surfaces clean & ventilated.
Blanket/Folding Panels	Portable cloth or semi-flex mat	Lays flat on ground; some airflow	-15 – 20 %	Raise slightly or tilt for airflow; avoid use on hot concrete or sand.

Optimising Summer Performance

Ventilation: Allow at least 20 mm airflow under fixed panels; shade flexible panels underneath if possible.
Tilt Angle: A 10–20° tilt improves convection and reduces stagnation.
Reflective Surfaces: Avoid mounting directly on dark metal; use light-coloured backboards.
Midday Operation: Charge batteries earlier/later in the day to sidestep peak heat.
Series vs Parallel: Parallel wiring can reduce MPPT clipping during derating.
Monitor Vmp & Imp: Use an MPPT controller display or multimeter to confirm expected voltage drop (typically 1–2 V per panel lower than STC).

Expected Output by Season (Example – 400 W Panel, -0.38 % / °C)

Region	Summer (40–45 °C ambient)	Autumn/Spring (25–30 °C)	Winter (15–20 °C)
QLD / NT	320 – 340 W	360 – 380 W	390 – 400 W
NSW / VIC	330 – 350 W	370 – 390 W	395 – 405 W
SA / WA North	325 – 345 W	365 – 385 W	395 W +

These figures represent normal, healthy performance — not a fault or warranty issue.

Quick Checklist for Customers

✅ Voltage drops more than 20 % in heat? → Check airflow or surface contact.
✅ Panels extremely hot to touch? → Normal under load.
✅ Output returns near rated W in cool morning? → Panel operating correctly.