Solar Panel Temperature Coefficient Calculator

See exactly how heat reduces your solar panel output each month. Compare HJT vs PERC vs budget panels — find out if upgrading to premium panels pays off in your climate.

Panel & location

Panel rated watts (STC)?

Temperature coefficient?

Location?

Utility rate?

¢/kWh

Temperature impact on annual production

756 kWh/year (8.6% heat loss)

Annual kWh loss from heat71 kWh

Dollar value of heat losses$9/yr

Monthly output (adjusted for temperature)

Month	Avg Temp	Panel Temp	Actual W	kWh
Jan	55°F	38°C	382W	58
Feb	59°F	40°C	379W	56
Mar	65°F	43°C	374W	67
Apr	74°F	48°C	367W	67
May	83°F	53°C	360W	71
Jun	92°F	58°C	353W	70
Jul	95°F	60°C	351W	66
Aug	93°F	59°C	353W	65
Sep	88°F	56°C	356W	62
Oct	76°F	49°C	366W	62
Nov	63°F	42°C	376W	57
Dec	55°F	38°C	382W	55

Panel technology comparison (annual kWh, 1 panel)

HJT (−0.26%/°C)775 kWh

TOPCon (−0.30%/°C est.)766 kWh

PERC (−0.35%/°C)756 kWh

Your panel (-0.35%/°C)756 kWh

Moderate heat losses. TOPCon or HJT panels would improve annual output by 5–10% in your climate.

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How to Use This Calculator

Enter your panel's rated watts (STC)

This is the nameplate wattage printed on your panel label — measured at Standard Test Conditions (STC): 25°C cell temperature, 1000 W/m² irradiance, and 1.5 air mass spectrum. It's the "peak power" rating you see in marketing materials. Real-world output in summer is often 10–20% lower because panels heat up well above 25°C.

Select your temperature coefficient

The temperature coefficient of maximum power (Pmax) is listed on every panel datasheet, typically expressed as %/°C. It tells you how much output decreases for every degree above 25°C. A coefficient of −0.35%/°C means a panel operating at 65°C (40°C above STC) produces 14% less power than its rated output:

HJT (Heterojunction) — Best-in-class: −0.26%/°C. Worth the premium in hot climates.
TOPCon — Very good: −0.30%/°C. Premium monocrystalline technology.
PERC — Standard modern panel: −0.35%/°C. Widely available, good value.
Budget polycrystalline — Older technology: −0.40%/°C. Avoid in hot climates.
Thin-film CdTe — Excellent for heat: −0.25%/°C (First Solar). Lower efficiency but superior temperature performance.

Select your location

The calculator uses monthly average ambient temperatures for your city and applies the NOCT (Nominal Operating Cell Temperature) model to estimate actual panel operating temperature: roughly ambient + 25°C for roof-mounted panels. Summer months in Phoenix can push panel temperatures to 70–75°C, causing significant derating.

The Formula

Panel operating temp (°C) = Ambient temp (°C) + 25°C [NOCT approximation] Delta T = Panel operating temp − 25°C [STC reference temp] Output factor = 1 + (Temp coefficient % ÷ 100) × Delta T Actual watts = Rated watts × Output factor Monthly kWh = (Actual watts ÷ 1000) × PSH × 0.85 × Days in month Annual kWh = Sum of all 12 monthly kWh values Heat loss = Annual kWh at STC temps − Actual annual kWh

The +25°C offset from ambient to panel temperature is a simplified NOCT model. The actual NOCT (Nominal Operating Cell Temperature) is specified on panel datasheets (typically 43–47°C) and represents panel temperature at 800 W/m² irradiance with 20°C ambient and 1 m/s wind. In real roof-mounted conditions with low wind and full irradiance, operating temperature can be 30–40°C above ambient.

The 0.85 system efficiency factor accounts for inverter efficiency (~96%), wiring losses (~2%), and mismatch losses (~2%). Monthly peak sun hours (PSH) vary by season — summer months have more irradiance hours, which is when heat losses are also highest.

Example

400W PERC panel in Phoenix, AZ — July

Phoenix July average temperature is approximately 95°F (35°C). What does the panel actually produce?

Panel rated output (STC)400 W

Temperature coefficient−0.35%/°C

Ambient temperature95°F = 35°C

Panel operating temperature35 + 25 = 60°C

Delta T from STC60 − 25 = +35°C

Output factor1 + (−0.35/100) × 35 = 0.8775

Actual output400 × 0.8775 = 351 W

Result

Output loss400 − 351 = 49 W (12.3% loss)

July PSH (Phoenix)7.1 hrs/day

July kWh (single panel)0.351 × 7.1 × 0.85 × 31 = 65.7 kWh

vs STC output0.400 × 7.1 × 0.85 × 31 = 74.9 kWh

Monthly heat loss9.2 kWh per panel

A 20-panel system would lose 184 kWh in July alone from temperature derating. If you switched to HJT panels (−0.26%/°C), the July output factor would be 0.909 instead of 0.878, recovering about 5.6 kWh per panel per month — roughly $13/month for a 20-panel system at $0.13/kWh.

FAQ

What is temperature coefficient for solar panels? ▼

Temperature coefficient of maximum power (Pmax) is the rate at which a solar panel's output decreases as cell temperature rises above 25°C. It's expressed as %/°C and is listed on every panel datasheet. A coefficient of −0.35%/°C means output drops 0.35% for every degree the cell temperature exceeds 25°C. At a cell temperature of 65°C (common in summer on rooftop systems), output is 14% below rated power. This is one of the most important specifications to compare when selecting panels for hot climates — yet it's often overlooked in favor of rated wattage.

Why do solar panels lose power in heat? ▼

Semiconductor physics. Solar cells are made of silicon, and like all semiconductors, higher temperatures increase electron activity in ways that reduce the band gap and lower the open-circuit voltage (Voc). The fill factor also decreases. The net effect is reduced maximum power output. This is counterintuitive to many homeowners who assume hot, sunny days are the best for solar — they have the most irradiance, but the heat partially offsets the extra sunlight. Cold, sunny days (like a clear February day in Denver) often produce higher hourly output than hot summer days.

Which solar panels are best for hot climates? ▼

For hot climates (Arizona, Florida, Texas, Southern California), prioritize panels with the lowest temperature coefficient: (1) HJT (Heterojunction) panels — −0.24 to −0.27%/°C, the current best available in crystalline silicon. Brands: REC Alpha, Panasonic EverVolt, Huasun. (2) TOPCon panels — −0.28 to −0.32%/°C, now widely available from Jinko, LONGi, Canadian Solar. (3) Thin-film CdTe (First Solar) — −0.25%/°C but lower efficiency (19–22%). Avoid budget polycrystalline panels (−0.40%/°C) in climates with summer ambient temperatures above 90°F.

What is NOCT and how does it differ from STC? ▼

STC (Standard Test Conditions) is the lab rating: 25°C cell temperature, 1000 W/m² irradiance, AM1.5 spectrum. NOCT (Nominal Operating Cell Temperature) is a more realistic outdoor measurement: 800 W/m² irradiance, 20°C ambient temperature, 1 m/s wind speed. The NOCT value (typically 43–47°C for most panels) tells you what cell temperature to expect in standard outdoor conditions. For actual installed roof panels in summer (less wind, higher irradiance), cell temperatures are often 10–15°C higher than NOCT. Some manufacturers now publish PVUSA Test Conditions (PTC) ratings which are 10–15% lower than STC — closer to real-world output.

Does roof color affect panel temperature? ▼

Yes, but less than you might think. Dark roofs absorb more heat, which warms the air under and around the panels. Studies suggest dark roofs raise panel operating temperature by 3–8°C compared to white or light-colored roofs. At −0.35%/°C, this represents a 1–3% additional output penalty. The bigger factor is airflow under the panels — racking systems that allow several inches of air gap between panel and roof dramatically improve cooling. Flush-mounted panels (no airflow) run 10–15°C hotter than well-ventilated installations. This is why most installers maintain at least a 3–4 inch gap between panels and the roof surface.