Google announced Wednesday that it will try to shake up the U.S. mobile industry with Project Fi, a service that starts at $20 per month and will run on pre-vetted WiFi hot spots as well as Sprint’s and T-Mobile US’s 4G LTE cellular networks. 

Taking aim at mobile services encumbered with restrictive or complicated data plans, Google said the $20 per month fee will cover basics such as talk, text, WiFi tethering and international coverage in more than 120 countries, plus a flat $10 per gigabyte for cellular data in the U.S. and abroad, or 2GB for $20, 3GB for $30 per month, and so on. Customers will also get money back each month for data they don't use. 

“Since it's hard to predict your data usage, you'll get credit for the full value of your unused data,” Nick Fox, VP of communications products at Google, explained in this blog post about the new service. “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.”

In the vein of so-called “WiFi first” strategies that are being pursued by some cable operators as well as providers such as Scratch Wireless and Republic Wireless (Cablevision Systems has launched a WiFi-only service called Freewheel), Google said Project Fi will securely auto-connect customers to open WiFi hotspots (Google said it has verified more than 1 million of them "as fast and reliable") or a partner's cellular network. Taking it a step further, Project Fi aims to connect customers to the fastest available network at a given location, whether that’s via WiFi or the Sprint or T-Mobile 4G network.

Google currently is limiting Project Fi to an invitation-only Early Access Program, and will initially offer the service on the Nexus 6, a Motorola-made device that is the first to support the service’s new WiFi/cellular handoff capabilities.