i'm N. facing, with a hill to the S.W., so with the ambient temp going down to -5-10C in winters, it doesn't do diddly. i drain it and shut the taps filling it from late oct. to mid april. even now, with sporadic spring clouds passing, i'm lucky if it raises the temp from 15C to 40C after all day, which is OK for a quick shower without any cold added.

alternatively lighting a small fire tips it up to very hot.

on a full sunny day, such as we had a few days ago, it went to 60C on solar.

 

~"When an inner situation is not made conscious, it appears outside as fate." Karl Jung~