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.
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.