Calculate Solar Yield: Three Methods Compared

How much electricity will your solar system actually produce? The answer depends on which method you use — and when in the system’s lifecycle you ask the question. This article compares the three key approaches: PVGIS (historical data), weather-based forecasts, and real-time monitoring.

Method 1: PVGIS — Historical Satellite Data

PVGIS (Photovoltaic Geographical Information System) is a free tool from the European Commission’s Joint Research Centre. It uses over 15 years of satellite-measured irradiance data to calculate expected annual yields.

Best for: Planning phase — system sizing, ROI calculations, financing decisions

Feature	Detail
Data source	Satellite measurements (15+ years)
Accuracy	±5-8% for annual yield
Timeframe	Long-term average (years)
Cost	Free
What it tells you	Expected annual and monthly yield at your location
Limitation	Cannot predict specific days or weeks

Method 2: Weather-Based Forecast

Weather-based solar forecasts use numerical weather prediction (NWP) models to predict irradiance for the coming hours and days. They convert weather data into expected PV output for your specific system.

Best for: Operational phase — daily self-consumption optimisation, appliance scheduling

Feature	Detail
Data source	Current NWP weather models + irradiance models
Accuracy	±10-15% for daily yield
Timeframe	Hours to days ahead
Cost	Free (Solantiq) to paid (commercial APIs)
What it tells you	Expected output today and tomorrow
Limitation	Weather forecasts become less accurate beyond 3 days

Method 3: Real-Time Monitoring

Monitoring systems measure actual output in real-time via your inverter or smart meter. They provide historical data, performance analytics, and fault detection.

Best for: Performance tracking — detecting issues, verifying expectations

Feature	Detail
Data source	Inverter sensors / smart meter
Accuracy	100% (actual measurement)
Timeframe	Real-time + historical
Cost	Usually included with inverter (free app)
What it tells you	Actual output, consumption, grid feed-in
Limitation	Only shows what happened, not what will happen

Method Comparison

Criterion	PVGIS	Weather Forecast	Monitoring
Data source	Satellite (historical)	Weather models (current)	Inverter sensors (real-time)
Accuracy	±5-8% (annual)	±10-15% (daily)	100% (measured)
Timeframe	Long-term average	Hours to days	Real-time + historical
Best phase	Planning	Daily operation	Performance tracking
Cost	Free	Free (Solantiq)	Included with inverter
Solantiq tool	Solar Calculator	Solar Forecast	—

When to Use Which Method

1. Before purchase: Use PVGIS (via our Solar Calculator) to estimate annual yield and payback period
2. After installation: Use weather forecast (via our Solar Forecast) to optimise daily self-consumption
3. Ongoing: Use monitoring to verify performance matches expectations
4. If yield seems low: Compare monitoring data against PVGIS reference — if actual yield is below 85% of PVGIS prediction, investigate