Modelling FAQ

Modelling FAQ

Frequently asked questions about Solcast Modelling

This page represents the most frequently asked questions that the Solcast modelling team receive. Questions about global horizontal irradiance, global tilted irradiance, direct horizontal irradiance, direct horizontal irradiance and why one value might be greater than another.

Frequently asked questions:

Why is my GHI value higher than GTI?

Higher GHI than GTI is a normal scenario. This happens when direct irradiance is so low such that the benefits of being tilted towards the sun are offset by the reduction in visible sky which decreases the amount of incident diffuse radiation. This phenomena is why smart tracking panels may rotate to a horizontal position under cloudy conditions to maximise the amount of sky they can see.

GTI can be referred to as the tilted irradiance.

Why is my GHI value higher than DNI?

GHI or DHI being larger than DNI is not an uncommon scenario. This happens under partially cloudy conditions when there can be little or no direct irradiance (so DNI is low or 0) but there is still some diffuse irradiance which causes GHI to be larger.