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☀️ Irradiation vs Irradiance

Clarify the difference between irradiance and irradiation, how they are used in solar performance calculations, and where each appears in the Dashboard and reports.

1) Irradiance (W/m²)

Definition

Irradiance is the instantaneous solar power received on a surface per unit area.

  • Unit: W/m²

  • A power measurement (like “speed”)

Characteristics

  • Varies every second (clouds, shading, angle)

  • Measured by a pyranometer or irradiance sensor

  • Used for real-time performance indicators

Where it’s used

  • Real-time dashboards

  • Alarm detection (sudden drops)

  • Model comparison at 1-min / 5-min intervals

Example

750 W/m² at 12:15 → the sun is strong at that specific moment.

2) Irradiation (Wh/m² or kWh/m²)

Definition

Irradiation is the total solar energy received over a period of time.

  • Unit: Wh/m² or kWh/m²

  • An energy measurement (like “distance travelled”)

Characteristics

  • Represents the integral of irradiance

  • Calculated daily, monthly, or yearly

  • Used for energy-based KPIs and reporting

Where it’s used

  • PR (instantaneous calculation)

  • Daily yield and performance summaries

  • Monthly reports

  • Long-term benchmarking (year-to-year comparison)

Example

4.6 kWh/m² today → total energy received from sunrise to sunset.

📌 Quick Comparison Table

Parameter Irradiance Irradiation
Meaning Instant power Total energy
Unit W/m² Wh/m² or kWh/m²
Type Real-time Accumulated
Source Pyranometer Integration of irradiance
Used for Monitoring, alarms Daily/monthly KPIs, reports
Frequency Seconds/minutes Day/month/year

🌞 Practical Impact for Solar Performance

  • A temporary cloud causes a drop in irradiance → immediate performance change.

  • The daily irradiation may still be high if the rest of the day was clear.

  • Irradiance helps detect anomalies like inverter mismatch in real time.

  • Irradiation is essential for yield assessments and contractual guarantees.

🧪 Example Scenario

At 11:00, irradiance drops from 600 W/m² to 200 W/m² → passing cloud.
At the end of the day, the irradiation still reaches 5.1 kWh/m² → sunny day overall.