Vast amounts of free energy rain down on your house all day.
For every 1m² there is 1kW of energy from the sun. So if you have 10m² of south facing window that's 10kw, and sunny for 8 hours- 10 * 8 = 80kWh thats a lot of free energy !
If it's been sunny all day and the heating is on ? You are doing something wrong !
Make the most of Passive solar where possible. Maximise the south facing windows by keeping shading to a minimum no net curtains or trees blocking light. Make sure to open curtains on sunny days.
In fact when your house well sealed and insulated & you have no summer shading you will need to manage the solar radiation manually with Insulated curtains or you can find the house overheating in the summer months or cold because you didn't open them.