Curtains and shades on windows help prevent your rooms from overheating in the summer and from losing heat in the winter. That translates into less dependence on A/C cooling or heating to maintain a comfortable temperature indoors. Which means using less electricity, and the fuels or generation facilities that provide that electricity. And burning less gas, if your home is heated by natural gas.
On a warm 90 F day and parked in the sun, the interior of a locked car can get to 140 F or hotter in just an hour. Often, crime concerns prevent people from keeping the windows partially open. Folding shades for car windshields and windows are cheap, and they reduce the sunlight through a car's windshield and windows, preventing the greenhouse effect heat buildup. So, you will not need to use as much A/C (and electricity or fuel to power it) when you enter your car.
-Mitul Sarkar USA
As part of the Small Steps, BIG IMPACT HeroX challenge