How to Calculate Solar Irradiance (W/m² to Peak Sun Hours)
Quick answer: How to calculate solar irradiance (W/m² → PSH) #
Daily insolation (kWh/m²/day) ≈ (Average irradiance in W/m² ÷ 1000) × daylight hours
At STC reference, 1 kWh/m²/day ≈ 1 peak sun hour (PSH) before site derate.
Example 1: 417 W/m² average over 12 h → 417/1000 × 12 = 5.0 kWh/m²/day → ~5.0 PSH.
Example 2: 500 W/m² average over 10 h → 500/1000 × 10 = 5.0 kWh/m²/day → ~5.0 PSH.
Irradiance (W/m²) is instantaneous intensity; sizing needs daily energy. Prefer plane-of-array (POA) when tilt is fixed; GHI-only data needs transposition for accuracy (use OEM/meteo tools).
Verify with Peak Sun Hours Calculator →
Best for: engineers turning weather/resource files into sizing inputs.
Not ideal for: replacing TMY weather-year simulations (PVsyst/SAM) or satellite API products.
What is irradiance vs insolation vs PSH? #
Conclusion: W/m² is power density; kWh/m²/day and PSH are energy metrics for sizing.
| Term | Unit | Use |
|---|---|---|
| Irradiance | W/m² | Instant intensity (sensor / clear-sky) |
| Insolation / irradiation | kWh/m²/day | Daily energy on a surface |
| Peak sun hours | h/day | Equivalent hours at 1,000 W/m² |
CTA: Convert average W/m² in the Peak Sun Hours Calculator (irradiance fields).
Example 3: Total solar irradiance over a day #
Given: Clear-sky mid-day peaks near 1,000 W/m², but the daylight average is only 350 W/m² over 11 h.
Daily insolation = 350/1000 × 11 = 3.85 kWh/m²/day ≈ 3.85 PSH before derate—not “11 peak sun hours.”
Use this PSH in Solar Panel Sizing or PV Energy Production; do not treat clock daylight as PSH.
How data enters CalcPanel tools #
- Get PSH → Peak Sun Hours
- Size array → Solar Panel Sizing / How to Size a Commercial Solar Array
- Annual kWh → PV Energy Production with losses from Solar System Losses & Derating
Tilt effects: Solar Panel Tilt Angle Explained · Panel Tilt Calculator.
Common mistakes #
- Treating daylight hours as PSH.
- Using GHI as POA on steep roofs without adjustment.
- Double-counting soiling in both resource and loss %.
- Summer-only irradiance for winter autonomy.
- Confusing peak irradiance (1,000 W/m²) with daily average used in the conversion.
Next step: Verify with the PSH calculator #
- Peak Sun Hours Calculator on the Solar calculators hub.
- Solar Panel Sizing · PV Energy Production.
- Losses guide for derate bands.
Assumptions and disclaimer #
Average W/m² conversions are planning screens. Bankable designs need measured or licensed weather datasets and transposition models.
FAQ #
How do I calculate solar irradiance for PV design?
Daily kWh/m² ≈ (average W/m² ÷ 1000) × daylight hours. Example: 417 W/m² × 12 h → 5.0 PSH before derate. Then enter PSH in array sizing or production tools.
How do I calculate total solar irradiance over a day?
Integrate or average irradiance across daylight, then convert: average W/m² ÷ 1000 × hours = kWh/m²/day. Do not multiply peak clear-sky W/m² by daylight hours.
What is the difference between irradiance and peak sun hours?
Irradiance is instantaneous W/m². Peak sun hours are equivalent full-sun hours at 1,000 W/m² matching daily energy.
Can I enter W/m² in CalcPanel?
Yes—the Peak Sun Hours Calculator accepts average irradiance and daylight hours to set insolation/PSH.
Do I still need system losses?
Yes. Resource (PSH) and equipment/soiling losses are separate—see Solar System Losses & Derating.