Cutting The Electric Bill

I have been hanging those automobile heat shields in my east or west facing windows at home to block heat from the sun. On an east facing door I hung the sheet outside between the storm door & door, well that one really worked, while others hung inside still let heat in. This AM I hung one outside the patio door & now that glass is cool to the touch just like the door's glass was. Plans are to try & put them between the windows & screens on the other widows. These would work on cars too, if they were outside rather than inside over the dash.