Window Air Conditioner Query

Hi. I'm able to cool my little house with one window air conditioner unit. Every year, I install and de-install the unit in the same window, which is a casement. As I get older and my bad back worsens, each year this becomes more difficult. I have a feeling that this year will be the last year I'll be able to horse that thing in and out of the window. I have seen window units in the windows of other houses that seem to live there all year, with covers on them. I'd love to learn how that works, how those windows with the units in them remain energy efficient all year round. Can you help? Thanks. Cats