However, neither the Nexus nor the Kindle Fire have enormous retina displays to power, so having double the battery doesn't necessarily mean double the battery life: Amazon claims 7.5 hours of video from its battery, Google says nine (eight in "active use"), Apple says ten and Microsoft isn't saying anything yet.
Nexus 7 vs new iPad vs Kindle Fire vs Surface: operating system
The Nexus 7 runs Jelly Bean, Android 4.1 with the Project Butter user interface, while the iPad currently runs iOS 5 (iOS 6 is out in a few months).
The Kindle Fire runs Amazon's own forked version of Android, while the Surface tablets will run Windows RT on the ARM-powered model and Windows 8 Pro on the Intel-powered one. The Surface RT also comes with integrated Microsoft Office for RT.
Amazon Kindle Fire review: interface
Google Nexus 7 review
New iPad 3 review: interface
Hands on: Microsoft Surface review
Nexus 7 vs new iPad vs Kindle Fire vs Surface: key features
All four tablets expect to be tied to the cloud, so Amazon offers free storage of all your Amazon-purchased content and unlimited movie streaming for Amazon Prime customers, Apple has iCloud/iTunes, and Microsoft has repositioned Windows as a cloud-connected OS.
Google, of course, has its various mobile services as well as Google Play, and many of its services are being overhauled for Android 4.1.
Google Nexus 7 review
Every tablet has integrated Wi-Fi, while both the Nexus 7 and the new iPad have Bluetooth. We're assuming Bluetooth is in the Surface tablets too. The Nexus also has Near Field Communications (NFC), although only the iPad offers (optional) 3G/4G mobile broadband.
The Kindle Fire is camera-free while the Nexus 7 has a front-facing camera for video calls. The iPad has front and rear cameras: a 0.3 megapixel one for FaceTime and an HD video camera for proper recording (although if you use it as a camera, people will point at you and laugh).
The Surface's twin cameras are supplemented with twin microphones, which Microsoft says are "tuned for Skype".
The two things that really differentiate the Surface from other tablets are its integrated kickstand and its Touch Cover and Type Cover, which manage to cram keyboards and trackpads into impossibly thin panels.
If they work - which nobody can be sure of just yet; all we've got to go on is Microsoft's promises - they could turn out to be Surface's USP.
Nexus 7 vs new iPad vs Kindle Fire vs Surface: Price
Let's go with US prices so we're comparing like with like here: the Kindle Fire is $199, the Nexus 7 is $199 (8GB) or $249 (16GB) with $25 of Play store credit, and the iPad is $499 to $699 for the Wi-Fi model and $629 to $829 for the Wi-Fi + Cellular models.
The UK prices for the Nexus are £159 for the 8GB and £199 for the 16GB.
Read our Amazon Kindle Fire review
Microsoft hasn't announced pricing for either Surface tablet yet, although it promises that the prices will be "competitive". We'd assume that means that the Surface RT will be competitive with the iPad and the Surface Pro with ultrabooks.
Both the iPad and the Kindle Fire (US only) are already shipping, and the Nexus will ship in mid-july. The Surface RT should ship around October, with the Surface Pro following three months later.
Nexus 7 vs new iPad vs Kindle Fire vs Surface: Target audience
The seven-inchers are all about consumption: the Kindle Fire is largely useless if you don't want to buy stuff from Amazon, while Google's presentation emphasised that the Nexus is "made for Google Play".
It's all about the screen and the content, just like the Kindle Fire, and Google laboured the Kindle similarities for so long - it does books! It does mags! It recommends stuff! - that we started wondering why Google didn't just call it the Gindle.
Read our New iPad review
There's more to the Nexus than just mimicking Amazon, however, and the Nexus is a very capable and very desirable tablet in its own right: where Amazon's emphasis is almost entirely on selling Amazon content, the Nexus wants to be your sat-nav system, your gaming device and maybe even your work machine too.
It's particularly significant outside the US, where so far the low-end tablet market has been one populated by fairly unremarkable cheapies and fire-sale failures. If Amazon doesn't hurry up and offer the Kindle Fire internationally, the Nexus is going to steal all its potential customers.
We can imagine some prospective iPad shoppers going for a Nexus too: it's much, much more polished than similarly-sized Android tablets, and Google's clearly put an enormous amount of effort into making it as effortless and welcoming as possible.
The killer feature here, of course, is the price: Google's offering what appears to be a premium product for the sort of money you'd pay for a no-name Chinese cheapie. If it's as good as it looks, it's going to sell by the truckload.
Where do the Surface tablets fit in here? Our gut feeling is that they don't. Unless Microsoft starts chucking serious money around, the Surface RT is too big and too expensive to slug it out in the cheapie market with the Nexuses and Kindle Fires; it's designed as an iPad rival, not a Kindle Fire one.
Meanwhile the Surface Pro is aimed at a different market altogether: it's an alternative to an ultrabook for people who want laptop/tablet hybrids. The Nexus isn't a threat to either tablet because it's not playing in that market.