Google is now a mobile carrier. Today the company has made official its plan to offer wireless service to owners of its Nexus 6 smartphone. It's called Project Fi, and Google is launching an early invite program beginning today. "Similar to our Nexus hardware program, Project Fi enables us to work in close partnership with leading carriers, hardware makers, and all of you to push the boundaries of what's possible," the company wrote in a blog post.

The service is only available for the Nexus 6 and requires a special SIM card for Project FI — it will work with both existing Nexus 6 devices and new ones. Google says that right now the service is only available as an "early access program," and during that program it won't work on other phones.

Google's new offering is unique in that the company will charge consumers only for the data they use rather than hit them with a flat monthly fee that comes with a preset amount of data. If you fail to use all the data you've paid for, Google will refund you the difference.

For $20 a month you get all the basics (talk, text, Wi-Fi tethering, and international coverage in 120+ countries), and then it's a flat $10 per GB for cellular data while in the U.S. and abroad. 1GB is $10/month, 2GB is $20/month, 3GB is $30/month, and so on. Since it's hard to predict your data usage, you'll get credit for the full value of your unused data. Let's say you go with 3GB for $30 and only use 1.4GB one month. You'll get $16 back, so you only pay for what you use.

If you go over your plan, Google will simply charge you at a pro-rated rate of $10 per GB. In other words, if you pay for data and don't use it, you get refunded. If you don't buy data and use it, you end up paying the same amount. There are no family plans available, but neither does it require a contract of any kind.

As reported previously, Google will operate its wireless service with the help of both T-Mobile and Sprint; customers will have access to both networks, and Google's service will intelligently switch between them and Wi-Fi to maintain strong reception. "We developed new technology that gives you better coverage by intelligently connecting you to the fastest available network at your location whether it's Wi-Fi or one of our two partner LTE networks," the company said. Project Fi also supports voice calls and texting over Wi-Fi, lending subscribers more flexibility and how and where they can communicate with their contacts. Google also says it's using secure tech (there's a key that shows up in your menu bar) for when you're using public Wi-Fi hotspots.

Project Fi phone numbers "live in the cloud," according to Google, enabling you to text and place voice calls from a laptop or tablet without your actual phone nearby. When you are on the phone, Google says calls can seamlessly transition to LTE when you leave a Wi-Fi network. Google seems to be using the new, combined Hangouts / Google Voice infrastructure in some way for Fi, as its FAQ references it often.

If you're interested in being part of Google's mobile experiment, the signup page is here. Google says it'll be sending out a small number of invites every week starting now.