Using the Advanced Solar HR mod and changing the mod setting to reduce the available tiers, on restart I get:
Failed to load mods: __add_flat_solar.lua:253:attempt to index local 'base_recipe' (a nil value)
stack traceback:

Flat-Solar/add_flat_solar.lua:253: in function 'flat_solar_find_tech'
Flat-Solar/add_flat_solar.lua:22: in function 'add_flat_solar'
Flat-Solar/data-final-fixes.lua:17: in main chunk
* ^^ leading double underscores not displayed here

Mods to be disabled:
Flat-Solar (0.1.8)

Note: If the default setting for the available tiers is left unchanged, no error occurs.

Thanks. Just uploaded a fix for that.

The error is gone, but still not quite right. Now if default (all tiers), all flat panels are available. However, if I reduce the tiers, no flat panels are available.

Uh. That's weird. My check was per-panel. Let me test and see

I am doing this testing in sandbox mode, if that would make a difference.

Ugh. No, I made a stupid. I shouldn't try to bugfix before coffee. Should be fixed in the latest version.

Looks good to me now, thank you very much.