>>48336
>>48337
Tested this thing quite extensively, since then rarely a day when I not put it in place. Can't do it outside now, too cold, and had lots of rain in September, so set up a place inside, behind window. Due to the parabola the light hits the barrel from ~8-9 to ~16, but way less hours fully.
In rain there is no way the water can get warm, not even lukewarm. Still it becomes warmer than fresh from the tap. It cannot take much from the room's temperature, because we had liek ~15 C in the house.
In slight overcast (no Sun peeking behind the clouds) it gets lukewarm. Still colder than my hand. Short intervals of direct sunlight can make wonders.
Full sunny day I can warm the barrel of water, but won't be as warm like it was during summer.

The question still is the winter Sun.