Does having the window open at home while the air conditioner is on cost you more?

If you are trying to cool your home or room to a temperature that is lower than the temperature outside, then the electricity needed will be much greater if the window is open. So, in short, yes.

AnswerOn a warm day, leaving your Windows open will allow warm air to replace the air that your air conditioner just worked to cool. Thus, your air conditioner will have to continue to run, which uses electricity, to re-cool this air. Of course, using more electricity will cost you more money.