Solar Panel Output Calculator

Calculate real-world panel output after temperature derating — see exactly how much your panels produce in your climate.

Your panel array

Number of panels?

panels

Panel wattage (STC)?

Temperature derating

Temperature coefficient?

Ambient (outdoor) temperature?

°C

Peak sun hours?

hrs/day

System losses (non-temp)?

Real-world output (20 panels at 400W, 25°C ambient)

10,283 kWh per year

Actual W/panel364W

Cell temp at full sun50°C

Temp power loss-9%

Daily production28.2 kWh

Monthly production858 kWh

STC-rated production11,300 kWh/yr

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How to Use This Calculator

Enter panel count and rated wattage

Start with your number of panels and their rated wattage (STC). The STC rating — found on the panel label and datasheet — is measured under Standard Test Conditions: exactly 25°C cell temperature and 1,000 W/m² irradiance. This is a lab condition that real panels rarely achieve outdoors, especially in summer heat.

Select temperature coefficient

The temperature coefficient of Pmax (often written as Pmax TC or TK Pmax) tells you how much panel output drops per degree Celsius above 25°C. Find this on your panel's datasheet — it's typically listed in the electrical specifications table. Standard mono PERC panels are around -0.36%/°C, while premium heterojunction (HJT) and back-contact (IBC) panels achieve -0.29%/°C or better. Lower is better.

Enter ambient temperature

Use your average summer temperature during peak production hours (typically 11 AM–3 PM). The calculator adds 25°C to your ambient temperature to estimate the panel cell temperature, which runs significantly hotter than the air around it due to solar radiation absorption. In Phoenix with 38°C air, cells can reach 63°C — a 38°C rise above STC, causing a 13%+ power loss.

Compare real-world vs. STC output

The result shows both your temperature-derated real-world output and the STC-rated output (what you'd get if panels never heated up). The difference is your temperature loss — and in hot climates, this can be a significant factor in why actual production is lower than the nameplate suggests.

The Formula

Temperature derating uses the standard IEC 61215 method:

Cell temperature = Ambient temp (°C) + 25°C (NOCT-based: panels run ~25°C above ambient at full sun) Temperature derating factor = 1 + (Temp coefficient / 100) × (Cell temp − 25°C) Actual watts per panel = Rated watts × Temp derating factor System kW (real) = (Panels × Actual watts) ÷ 1000 Daily kWh = System kW × Peak sun hours × Other losses Annual kWh = Daily kWh × 365 Example (400W panel, -0.36%/°C, 35°C ambient): Cell temp = 35 + 25 = 60°C Derating = 1 + (-0.36/100) × (60 - 25) = 1 - 0.126 = 0.874 Actual output = 400W × 0.874 = 350W per panel (-12.6%)

This is why solar proposals from installers often show production slightly lower than what you'd calculate from STC ratings alone. Temperature derating — combined with inverter losses, soiling, and shading — is why a "10 kW system" rarely produces 10 kW × peak sun hours × 365 days. The 86% system efficiency default in many calculators bundles all these effects together.

This calculator separates out the temperature effect so you can see it explicitly, which helps you compare how different panel technologies (mono PERC vs. HJT vs. IBC) perform in your specific climate.

Example

Standard vs. premium panel in Arizona heat

Comparing a standard monocrystalline PERC panel (-0.36%/°C) with a premium HJT panel (-0.29%/°C), both 400W, in Phoenix where summer ambient temperatures average 38°C.

Panels20 × 400W each

System size (STC)8.0 kW

Ambient temperature38°C

Cell temperature63°C

Peak sun hours6.5 hrs/day

Standard PERC (-0.36%/°C)

Temp derating-13.7%

Actual W/panel345W

Annual production (86% eff.)~13,680 kWh

Premium HJT (-0.29%/°C)

Temp derating-11.0%

Actual W/panel356W

Annual production (86% eff.)~14,115 kWh

The HJT panel produces 435 kWh more per year (3.2% more) simply from its better temperature coefficient. Over 25 years at $0.15/kWh, that's about $1,630 more savings from temperature performance alone — not counting the premium HJT panels' higher efficiency. In hot climates, temperature coefficient is a meaningful spec worth paying attention to.

FAQ

Why do solar panels produce less power in summer heat? ▼

Solar panels are actually semiconductor devices, and like most semiconductors, they become less efficient as temperature rises. The physics involves increased electron-hole recombination at higher temperatures, which reduces the open-circuit voltage (Voc) and thus power output. A panel rated at 400W at 25°C might only produce 345–365W on a hot summer afternoon in Phoenix or Las Vegas — despite having excellent sunlight. This is counterintuitive: the sunniest, hottest days have more solar resource but actually lower panel efficiency due to heat.

Are premium panels with lower temperature coefficients worth the extra cost? ▼

In hot climates (Arizona, Nevada, Florida, Texas), yes — the economics are often compelling. HJT and IBC panels with -0.29%/°C vs. standard PERC at -0.36%/°C produce 2–5% more energy annually in hot-summer regions. Over 25 years, that adds up to meaningful extra production. In cooler climates (Pacific Northwest, Great Lakes), temperature rarely rises enough to make a big difference, and the premium panel cost may not be justified by temperature performance alone. Other factors like efficiency rating, degradation rate, and warranty terms may matter more in those regions.

How hot do solar panels actually get on a sunny day? ▼

Solar panel cell temperatures regularly reach 55–70°C (131–158°F) under full summer sun. The NOCT (Nominal Operating Cell Temperature) rating on panel datasheets is typically 44–47°C at 20°C ambient and 800 W/m² irradiance. Under peak summer conditions with 35°C ambient and 1,000 W/m², cells can easily hit 60–65°C. The rule of thumb used in this calculator (ambient + 25°C) is a conservative estimate appropriate for average conditions. Some mounting configurations with poor ventilation can push cells even hotter.

Do solar panels perform better in winter? ▼

Per unit of sunlight received, yes — cold panels are more efficient than hot ones. On a clear winter day in Denver (0°C ambient), a 400W panel with -0.36%/°C runs at about 15°C cell temperature — 10°C below STC — meaning it produces about 3.6% more than its rated output per unit of sunlight. But winter has far fewer total sunlight hours, so despite the per-photon efficiency advantage, winter total production is still much lower. Snow is the other winter factor — a fully snow-covered panel produces nothing until it clears, but since panels are tilted and black, snow often slides off or melts within hours of sun return.

What's the difference between this calculator and the Solar Power Calculator? ▼

The Solar Power Calculator uses a single efficiency factor (86%) to represent all real-world losses combined. This calculator separates out the temperature component explicitly, letting you see how much your specific climate and panel technology affect output. It's the more precise tool — useful for comparing panel options, understanding why production varies by season, or getting accurate output predictions for hot-climate installations. For most homeowners planning a system, the Solar Power Calculator is sufficient. Use this one when you want to dig into the thermal performance specifically.