Do you recall your Grandma’s house, or maybe your ancient aunt so-and-so’s house? Remember how they had those funny bean bin type animals with long, outstretched paws, resting down at the bottom of the doorways? Periodically, they were resting on the window sill at the bottom of the window, as well, and well, although hardly anyone has those draft busters anymore, and they may look old fashioned (and I guess ARE old fashioned) they actually absolutely can help with the costs of running your heating and cooling system.
If your furnace or a/c has to always be on to heat and cool the air that keeps coming in through the cracks crevices and so on around windows plus doors, that furnace or a/c has therefore to run a lot more frequently.
The more often your a/c engages, the higher up your utility bill goes. In fact, anything at all that you can do to help your heating plus cooling set up spend more down time ie not on, the better off you will be. With me, for example, I do not use the oven in the summer. The extra heat of the oven, added on to the natural heat of the outside, is just too much for my exhausted ancient a/c to deal with. If I ran the oven, I bet my a/c would never shut off, so I just use the microwave. In the summer I also put up blackout curtains on my windows. When I close my curtains, the sunlight is completely plugged off. out. It’s never too hot in my condo in summer, and so my A/C doesn’t have to fight the hot sun. And in the winter, I open those drapes plus let the warmth of the sunlight help my heater.