Here is a summary of thing that we can expect from Windows on ARM according to a Microsoft blog on the matter:
Summary is from “Microsoft gets it right with Windows 8 on ARM, and why Apple should be worried" by Adrian Kingsley-Hughes
- ‘Out of the box’ experience will look and feel just like Windows 8 on x86/x64 hardware.
- The Windows desktop will still be available on WOA devices.
- Tools such as the Windows File Explorer and Internet Explorer will be available.
- PC makers will ship WOA devices at the same time as Windows 8 PCs ship.
- WOA will include Microsoft Word, Excel, PowerPoint, and OneNote.
- WOA will not support any type of virtualization or emulation approach, and will not enable existing x86/64 applications to be ported or run because of the adverse effect this would have on battery life.
- Microsoft is working to deliver WOA PCs that will combine three features: Thin and light in industrial design, long battery life, and integrated quality.
- WOA PCs will be clearly labeled and branded so as to avoid customer confusion with Windows 8 on x86/64.
- WOA will not be available as a software-only distribution.
- A WOA PC/device will feel more like a consumer electronics device rather than a traditional PC.
See the bullet in bold and italic? Just the fact that Windows will be running on ARM will affect the battery life. Simple as that.