Conspicuous in their absence in today's Microsoft Surface reveal were any estimates for battery life of the two models. The reason why is almost certainly a boring one: Windows 8 is still in pre-release form. Microsoft will not give a battery life estimate for an OS that isn't finished. It could very well change between now and when it goes final, so it makes perfect sense not to toss out a guess now.
The online spec sheet gives a clue, as it lists the battery size for the devices. The smaller ARM model has a 31.5 Watt-hour battery, while the larger Intel model has a 42 W-h battery. For reference, the new iPad has a 42 W-h battery. That's the primary competition for the Surface tablets, so that's going to be my comparison point here.
The Surface tablets have larger displays than the iPad does, but they don't have as many pixels as their rival from Cupertino does. That could mean battery savings that help the smaller Surface batteries (42 W-h on Intel counts as smaller than 42 W-h on ARM) get life similar to the iPad's "up to 10 hours" rating.
The issue is a bit more complex than that though.
Windows 8 does a lot more than iOS does, and both OS vendors are happy with that fact. It does make predicting battery life for any Windows 8 device by looking at the iPad difficult. Consider:
- Different app models. Metro apps get paused in the background like iOS apps do. If you stay in Metro full time, then your battery life on these Surface tablets will approach the maximum possible. However, Desktop apps do not get paused in the background. They will consume some amount of battery life if they're doing anything at all. If you, say, leave a Desktop browser displaying a Flash ad running in the background, that's going to be a big battery hit. Put the Metro version of IE10 displaying a Flash ad in the background and it gets paused, thereby avoiding a battery hit. iOS has nothing analogous to the Desktop.
- The Touch Cover and Type Cover. I am still trying to figure out exactly how they work, but as best as I can determine, they will be some kind of battery hit. They don't appear to have their own power sources, so they will have to draw on the Surface's battery. The iPad's external keyboard has its own batteries, but it still hits the battery some because the tablet has to run Bluetooth to connect to it. I don't know how comparable running Bluetooth is to powering one of these keyboard covers, but my guess is the keyboards draw more power.
These are the obvious ways in which battery life can and will diverge between the Surface tablets and the reference point iPad. I wouldn't be surprised to see the Surface machines get two battery ratings, just like how your car gets two fuel economy ratings. They might get a Metro-only rating and a mixed use (Metro/Desktop) rating because I think the power draw will be that big a difference. They might also get a caveat, something like "expect battery life to decline 5-10% while using a Touch Cover or Type Cover".
Ultimately, the real world battery life on these things will depend heavily on what inhabits your particular real world. If you go with a naked tablet and only use Metro apps, then you'll max it out. If you keep the keyboard cover on at all times and frequently use Desktop apps, then your battery life will decline. Microsoft is probably still deciding exactly how to rate the battery life on these devices because the possibilities are quite wide.
EDITED TO ADD:
After posting this, I noticed a good comment in another thread that the Intel Surface is probably more of a MacBook Air competitor than an iPad competitor. I think that's apt given that those both have Intel inside.
The 11" Air has a 35 W-h battery and its rated for 5 hours, and its screen is almost the same physical size as the Surface's. The screen on the ARM Surface is 1366x768, same as the 11" Air; I've seen rumblings around here that the Intel Surface has a 1080p screen.
Based on that quick comparison, I wouldn't be surprised if the Intel Surface is rated for higher battery life than the 11" Air. It has a larger battery, but also a higher screen resolution if the 1080p talk is correct. Both run Flash, both have apps that run all the time, and both have keyboards that draw power. It's a far more direct comparison than ARM Surface vs. iPad. If you stick to Metro only, you'll probably get more battery life than the 11" Air with ease.