But since modern buildings usually have huge windows, we're letting in about 1kw of heat from sunlight per square meter of window. That's 6kw for a 3x2m bay window, or about the equivalent of 6 electric heaters at full power.

Block that heat ouside of your house with all kind of shade you can get: trees, blinds, adjustable shades, rolling shutters...

Also, close every openings during the day, vent at night.