Solar Output by Irradiance Calculator

Frequently Asked Questions

How does irradiance affect panel output?

Output W ≈ STC rating × (irradiance ÷ 1,000 W/m²) × (1 − temp coefficient × (Tcell − 25 °C)). A 400 W panel at 800 W/m² and 45 °C cell with −0.35 %/°C produces 400 × 0.8 × (1 − 0.0035 × 20) ≈ 298 W.

Why does cell temperature reduce output?

Crystalline silicon loses about 0.3–0.45% of power per °C above the 25 °C STC reference. On a hot roof (cells often 25 °C above ambient), a 35 °C day puts cells near 60 °C, costing 10–15%.

What irradiance value should I assume?

STC uses 1,000 W/m², but real-world midday clear-sky values are typically 800–1,000 W/m². Use NREL's NSRDB or PVWatts hourly data for accurate site-specific output modeling.

Do bifacial panels change the formula?

Yes - bifacial modules add rear-side gain (typically 5–15% depending on ground albedo and mounting height). Multiply front-side output by a bifacial gain factor (1.05–1.15) for a rough annual estimate.

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How This Calculator Works

A solar panel's nameplate watts (e.g. 400 W) is its output under STC: 1,000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum - a laboratory ideal. Real output scales linearly with incoming irradiance (half the sun gives half the watts) and drops as the cell heats up - silicon panels lose roughly 0.3–0.5% per °C above 25°C. On a 35°C summer afternoon, the cell itself sits 20–30°C hotter than ambient, easily 55–65°C, eroding 12–18% of the nameplate rating. This tool combines instantaneous irradiance with cell temperature and the panel's temperature coefficient to estimate live output (W), and optionally projects daily Wh from peak sun hours.

Formula / Method

Instant W = nameplate W × (irradiance ÷ 1000) × (1 − tempCoef × (cellTemp − 25°C)), where tempCoef is the panel's power coefficient expressed as a decimal fraction per °C (a nameplate spec of −0.38%/°C is entered as 0.38). Cell temperature is roughly ambient + 25°C in still air at full sun, or use the panel's NOCT (Nominal Operating Cell Temperature, typically 45–48°C) for design-load conditions. Daily Wh = instant W × peak sun hours; this approximates a flat irradiance day, which is conservative because panels gain efficiency in cooler shoulder hours. Irradiance enters W/m²: clear midday is ≈ 1,000; bright overcast 200–500; deep overcast 50–150.

Worked Example

A 400 W panel at 850 W/m² irradiance, cell temperature 45°C, coefficient 0.38%/°C. Irradiance fraction = 0.85. Temperature loss = 0.38 × (45 − 25) = 7.6%, so temperature factor = 1 − 0.076 = 0.924. Output = 400 × 0.85 × 0.924 ≈ 314 W, or 78.6% of nameplate. Over 4.5 peak sun hours (with that irradiance and temperature as a daily average), daily energy ≈ 314 × 4.5 = 1,413 Wh = 1.41 kWh per panel. A 10-panel array would deliver ≈ 14 kWh/day under those conditions - matching field expectations far better than "10 × 400 W × 4.5 h = 18 kWh," which is the naive nameplate calculation that disappoints new owners.

Common Mistakes

Using ambient temperature for cell temperature - cells run 20–30°C hotter than ambient under full sun.

Confusing the power coefficient (%/°C, typically −0.30 to −0.50) with the voltage coefficient (V/°C) - both appear on datasheets but mean different things.

Assuming irradiance is a flat 1,000 W/m² for the "peak sun hours" window - PSH is an integrated quantity; instantaneous irradiance varies through the day.

Forgetting bifacial gain - bifacial panels on light-coloured ground can collect 5–15% extra from rear-side reflection.

Ignoring partial shading - a single shaded cell can cut a whole string's output disproportionately due to bypass-diode bypassing of full sub-strings.

Tips & FAQ

What is NOCT? Nominal Operating Cell Temperature, the cell temp under 800 W/m², 20°C ambient, 1 m/s wind - a more realistic test than STC. Typical NOCT is 44–48°C; a panel with lower NOCT runs cooler at given conditions and produces more. Why temperature matters: a desert site can lose 15–20% to heat; a cool, breezy alpine site at the same PSH can outperform nameplate. Panel chemistry: typical mono-PERC has a power coefficient of about −0.35%/°C; TOPCon and HJT are slightly better (−0.27 to −0.32%/°C); older poly-crystalline is worse (−0.45%/°C). Mounting: ground-mount arrays with airflow under the panels run cooler than flush-mount roof installs - budget +3–5% production for ground mounts. Cleaning: heavy soiling can cost 5–15% - rain handles most of it on tilted arrays; deserts need annual washes. Spectrum and elevation: high altitude (more direct, less diffuse) and clean cool air boost output noticeably - alpine arrays often outperform sea-level arrays at the same PSH. Cloudy-day output: at 200 W/m² (heavy overcast) you still get 20% of nameplate - not zero. Panels work in diffuse light. For accurate seasonal totals, use NREL's PVWatts which integrates real TMY irradiance, ambient temperature, and wind across 8,760 hourly steps; this calculator is for understanding instantaneous behaviour and rapid sanity checks.

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