Macbook Air 13 inch (256 GB/ 8 GB/ i5) along with Windows (Using BootCamp)
If you are truly dual booting (using Boot Camp) you will find your battery life better on OS X than on Windows. However, when you boot into OS X, you won't suffer any ill effects from having a separate Windows partition sitting idle. 

If you opt to run Windows in a Virtual Machine (using, for example, Parallels Desktop, VMWare Fusion, or Virtualbox), you will notice a significant decrease in battery life because you'll be running both "machines" (OS X and Windows) at one time.