To include Solar Collectors in your Part L calculation you will need to ensure that all the necessary data has been entered. The Solar Collector must be specified and attached to a room/zone, and then the surface data is needed so the area and orientation can be taken into account.
Solar Collectors should not be confused with Solar Panels, ie for Hot Water Systems.
1 - Define your collectors in the General tab:
2 - Assign to rooms/zones in Room Data tab:
3 - Specify containing wall and area in Misc tab:
The solar collector should then impact upon your results.