Google is now a mobile carrier, with the announcement of its new Project Fi service. Available exclusively to owners of Google’s Nexus 6 phone, Project Fi is a radical departure from the kind of wireless plans offered by leading carriers like AT&T and Verizon.

Unlike a standard plan, which charges you a flat rate for a set amount of data even if you don’t use all of it, Google’s plan will charge you only for the data you use and pay you back for any that you don’t.

Say, for example, that you purchase a 2 GB monthly plan for $20. If you use only 1.6 GB of that data, Google will reimburse you $4. Use only 0.4 GB of data and you’ll get $16 back.

In addition to data, Project Fi provides unlimited talk and text, unlimited international text, and coverage in more than 120 countries. (That base level of service costs $20 per month; you can then add a data plan for $10/GB per month).

And unlike most wireless plans, it charges you the same amount for data overseas as it does here in the U.S. Google will also let you use your phone as a Wi-Fi hotspot at no additional charge.

Beyond offering a new way to pay for your data, Project Fi has a unique way of connecting you to the Web. Rather than using single carrier network, Project Fi uses two: Sprint and T-Mobile.

So if you’re riding the bus on your way to work and your 4G LTE connection with Sprint starts to slow down and there is a faster T-Mobile signal available, Project Fi will automatically switch you over to T-Mobile’s network to ensure you always have the best possible connection.

If your Sprint and T-Mobile connections are running slow, Project Fi will move you over to one of more than a million free open Wi-Fi hotspots to get your data. While you’re connected to the hotspots, Project Fi automatically encrypts your information to ensure it remains private.

And because Project Fi uses Google Voice, you can take your phone number to any device you are signed in to with Google Hangouts.

There’s one big problem with Project Fi, though: It uses the two smallest of the Big Four U.S. carriers, Sprint and T-Mobile. As the carriers have smaller LTE coverage areas than AT&T and Verizon, you may have trouble getting an LTE connection when away from major cities.

Still, as it does with its 4G LTE connections, Project Fi will automatically switch you between T-Mobile and Sprint based on which network has the faster 3G connection.

T-Mobile’s outspoken CEO, John Legere, praised Google’s Project Fi for the way it could change the U.S. wireless market, though he was quick to point out that while Project Fi will benefit from T-Mobile’s network, it won’t get all the same features the carrier offers its own customers, including HD Voice.

It will be interesting to see if Project Fi has any impact on the wireless industry as a whole. Its pricing structure could prove incredibly helpful for wireless shoppers. So far, T-Mobile has been leading the charge in changing the wireless industry with its UnCarrier initiative, which has eliminated two-year service contracts and lets you roll over your unused data from month to month.

AT&T, Sprint, and Verizon have aped a handful of T-Mobile’s practices as the carrier has gained more customers. Could Google’s Project Fi be the beginning of a large sea change in the industry? We’ll have to wait to find out.

You can request an invitation to the service here.

Email Daniel Howley at dhowley@yahoo-inc.com; follow him on Twitter at @DanielHowley or on Google+.

