There's nothing like geting things for free. I'm a big fan of free heat in winter.
Every square metre of glass hit directly by the sun delivers about 1kWh of heat. That's like having a one bar radiator (old school style heating) in the room. For free!
To be clear - the sun has to hit the windows for the heat to get in, It's through the East facing windows in the morning, the North facing ones during the day and for the winter, those West facing ones actually are a small bonus.