Why can't you run desktop applications on Windows RT? If you look back at the picture of Windows running on an ARM smartphone, way back when Microsoft began experimenting with moving the Windows code to the ARM chip, you don't see a WinRT app. You see Solitaire — a familiar desktop app. If the desktop, a Windows application, can run and if Office, a (significantly rewritten) desktop app, can run, then why not other desktop apps?

Because they'd ruin your battery life — not to mention bringing all the malware that's written to attack Windows to a brand new platform.

As ex-Windows chief Steven Sinofsky put it when first announcing that Windows would run on ARM: "Our job is to allow the power, the flexibility and the choice of that architecture to shine through. If a processor package uses less power, the role of Windows is to let that reduction in power shine through."

Running desktop applications, nice as the idea is, wouldn't give you that. "Those apps don't take advantage of all the things that make ARM a unique offering. When you use an ARM chip you want great power management; in Windows 8 that's Connected Standby. An x86 app doesn't respect that and it's going to just drain the battery", said Sinofsky.

Anything you think is a compromise with Windows RT is almost certainly sacrificed to battery life.

Even more bluntly, at Build in 2011 he explained that "we decided early on that unless we could deliver the value proposition of that ARM hardware there was no point". Running x86 apps in emulation? "It would just undermine this whole thing." After all, an iPad doesn't run OS X apps — or as he put it, a little more subtly: "If you do run everything, all of a sudden [Windows on] ARM is saddled with all this stuff competitive platforms don't get saddled with."

But it's more than just having long battery life. It's about having what Pat Stemen (who's been working on power management in Windows since at least Windows 7) calls "consistent, consistently long battery life". The idea is that there shouldn't be anything that can fire up in the background and start using power, so if you know how large the battery is you know how long the battery will last and that doesn't change from day to day. No surprises.

That's why the thing Microsoft is most protective of on Windows RT is battery life, and anything you think is a compromise is almost certainly sacrificed to battery life.

Surface RT: see how it runs

That certainly delivers on the Surface tablet: you never really turn Surface off, especially if the Wi-Fi is on, and the battery just keeps going. On our last trip to the US, I was using my Surface on a Monday and I'd had it plugged in at some point that day. I used it a little on the Tuesday then flew back to the UK late afternoon. I pulled my Surface out of my bag on the Wednesday after we landed and used it a little. I used it more on the Thursday and the Friday and the Saturday afternoon. And sometime late Saturday night I finally had to plug it in. Now I wasn't using it all day long, but I was turning to it every hour or so to catch up online or read web pages and it was online pretty much all the time. My phone doesn't even give me that kind of battery life. Surface does it by turning the processor off any time it possibly can, leaving just the Wi-Fi running to accept incoming messages like email.

There are obvious design decisions in Windows RT that have nothing to do with battery life, like big tiles for touchscreens and contracts between apps that give you a richer version of the clipboard and the settings charm so you always know where the settings are.

Not joining the domain or having GPOs (Group Policy Objects) is because a Windows RT tablet is mine, not the IT team's and I don't want someone else locking down bits of my experience because they don't think I can be trusted to play Minesweeper and get my job done as well. Plus, doing away with GPOs makes it much harder for those 'fixing' utilities that tweak system settings that do nothing but break your system. I spent five years writing a Windows XP problem page and once service-packing XP stopped it being the most malware-ridden platform I've ever seen, the vast majority of problems were caused by having run a tweaking utility in the first place (turning off services to save memory and then finding the features that rely on them don't work is making a rod for your own back).

But given how much longer a badly-written log-on script can make starting up Windows, not having GPOs might make battery life more consistent as well.

And when it comes to the familiar desktop and the places it's not quite the same on Windows RT, the things you sacrifice are mostly things that would turn the processor back on.

Windows RT can join a homegroup, but you can't share content with the homegroup — just access the files on other computers. That's one of the clearest pointers that Microsoft sees Windows RT as something you use with a PC, not something that replaces a PC. But it also makes sense. If you're sharing content from a device, you want that content to be available whenever that device is on, but Windows RT is on all the time even when the screen is off. If other PCs could wake it up to stream music or view photos stored on your tablet, it would hammer the battery life.

Microsoft sees Windows RT as something you use with

a PC, not something that replaces a PC.

The same is true of offline files. Although offline files is based on a service and services can be throttled (that's why they're the only part of the Windows desktop that runs in Connected Standby), it means Windows regularly polling the server to look for changes to sync down to the PC (over and above uploading the changes I make locally at intervals). It's a hugely useful feature, and not having it is one of the main reasons Windows RT won't be my daily travelling PC (the other is that I need active pen support so I can scribble down notes), but it's the wrong kind of connection pattern for a modern battery-efficient system.

Sync in Windows RT is perfectly possible; after all, it syncs my Windows settings and my Wi-Fi and website passwords. (I followed a link on Twitter today to a site I hadn't visited in a year, but my Surface had the password because a year ago or more I saved it on some other PC that I've since upgraded to Windows 8 and logged into with the same Microsoft account). Apps can sync information — I get push email and calendar updates and instant messages — but that's all push rather than the pull model of offline files. And OneNote syncs its notebooks quite happily to Windows RT, both the desktop and Windows Store versions.

Syncing from SkyDrive

Given that the settings sync in Windows RT is information actually stored on SkyDrive, as are my OneNote files, I'm still hoping it's going to be possible for a future version of the Windows Store SkyDrive app that comes with Windows RT to sync my other files from SkyDrive as well — especially now the SkyDrive API supports selective sync. Until we had selective sync, I didn't want the relatively tiny storage space of a tablet filling up with everything I have on SkyDrive. But now I could pick and choose what folders to sync, I think it should be an option.

If it isn't, I won't say Microsoft hasn't been thinking things through or doesn't get what users want. I'll say that whenever there's a trade-off to be made between features and battery life, Windows RT chooses battery life.