Hi Aly,

It is better to plot a 'sunny day' (less cloud interference) to compare the actual vs TMY or a few continuous days. This shift could be possible due to two reasons:

1) Locations are slightly different

2) Local interference during the measurement period while TMY averages it out over the year.

Meteonorm is a commercial source for availing future climate data. There could be other services available depending on the region.