GTI (global tilted irradiance)
Radiation & sun Global tilted radiation tilted-plane irradiance GTI
GTI describes solar radiation received by a tilted surface such as a solar panel. The surface’s angle and orientation must therefore accompany the value.
Most roofs are not horizontal
Global radiation is usually given for a horizontal surface. A roof may slope and face east, south or west. Global Tilted Irradiance, or GTI, calculates radiation for that particular tilted surface.
Three contributions reach the panel
- Direct sunlight.
- Diffuse light from the sky.
- Light reflected by the ground.
Power is given in W/m². On our pro-data page, hourly values average the preceding hour. To obtain radiant energy, power must be accumulated over time.
The angles are part of the answer
A tilt of 0° is horizontal; 90° is vertical. An east-facing roof catches morning light differently from a west-facing roof in the afternoon. Without a specifically chosen tilt, this website’s current data request uses the provider’s horizontal default.
An estimate, not an electricity meter
Open-Meteo assumes 20 per cent ground reflectance and a simplified, evenly distributed diffuse sky. Real shading and reflections may differ. Efficiency, panel temperature and system losses also affect electricity production. Irradiance of 800 W/m² therefore does not automatically mean 800 watts of electricity per square metre.