text: Google launched a new U.S. wireless service today that switches between Wi-Fi and cellular networks . Google is already the world’s most popular phone software provider, and a pay-TV operator - and now it wants to be your mobile network carrier. The company has unveiled a U.S. wireless service that switches between Wi-Fi and cellular networks to curb data use and keep phone bills low. The service, called 'Project Fi,' debuted today, about two months after Google revealed its plans to expand its ever-growing empire into providing wireless connections for smartphones. Google is selling the basic phone service for $20 a month and will only charge customers for the amount of cellular data that they use each month, instead of a flat rate. Each gigabyte of data will cost $10 a month. That means a customer could sign up for a plan offering three gigabytes of data and get $20 back if only one gigabyte was used in a month. Most wireless phone carriers allow their customers to roll over unused data into another month of service without refunding any money. Project Wi-Fi initially will only be sold to a narrow US audience that owns the Nexus 6, a smartphone that Motorola Mobility made with Google's help. Google's pricing setup makes Project Fi less expensive than most of the comparable plans offering by the four biggest wireless phone carriers - Verizon, AT&T, T-Mobile and Sprint. The monthly prices for a single line of smartphone service with up to one gigabyte of cellular data at those carriers range from $45 to $50 compared to $30 from Google. The major carriers, though, offer a variety of family plans that could still be better deals than Project Fi. Those bundled plans allow several phone lines to share a pool of cellular data. Scroll down for video . Besides trying to bring down the prices of wireless phone plans, Google is promising subscribers that their Nexus 6 model will automatically connect with the fastest network available. Rather than building its own network, Google is leasing space on cellular towers built by Sprint and T-Mobile, which are hoping the deals will boost their profits without costing them too many customers tempted to defect to Project Fi. To use the service, Nexus 6 owners must sign-up to request an invitation, must have a Gmail address, and must live in a US zip code within the coverage area. In this map, the dark green areas are covered by the service's 4G LT, the lime green is covered by 3G and the pale green by 2G . Verizon 4G LTE coverage is pictured for comparison. Project Fi will be hosted through Sprint Corp and T-Mobile's networks . The service will work only on the company's Nexus 6 phones and only in the US . Project Fi will be hosted through Sprint Corp and T-Mobile's networks. Overall, it costs $20 for basic service, which includes unlimited domestic talk, unlimited texting, tethering, and access in 120 countries. Customers pay $10 per GB of data. Data is paid for in advance, and the cost of unused data gets refunded rather than rolled over or lost. Phone numbers will live in the cloud so that consumers can talk and text on any connected tablet. Calls can be made via Hangouts on Android, iOS, and through Gmail on desktops, via the Hangouts widget. Texts can be made and received in the same way. There is no annual service contract required when you sign up . To use the service, Nexus 6 owners must sign-up to request an invitation, must have a Gmail address, and must live in a US zip code within the coverage area. Google is promising Project Fi will automatically switch over to an available Wi-Fi network if that is running at a higher speed than the cellular alternatives. T-Mobile CEO John Legere, whose company already has been cutting its prices and rolling out new options, said it was a 'no-brainer' to work with Google on Project Fi. 'Anything that shakes up the industry status quo is a good thing — for both US wireless customers and T-Mobile,' Legere wrote in a blog post. Google has an incentive to promote cheaper and faster wireless service as a mobile virutal network operator. This is because it operates some of the world's most popular online services, including its search engine, maps, Gmail and YouTube video site. The Mountain View, California, company believes most people will visit those services more frequently if they are enticed to stay online for longer periods, giving Google more opportunities to show the digital ads that generate most of its revenue. Similar motives prompted Google to begin building high-speed, hard-wired networks capable of navigating the Internet at speeds up to 100 times faster than existing broadband services. Although Google is only selling its broadband service in a handful of U.S. cities so far, AT&T and Comcast are now offering options with comparable speeds in a few communities. Price in Dollars: This graphic shows how much Google's new Project Fi will cost for various plans. For Talk and Text only, the Google carrier will cost $20 a month as compared to $35 for new partner TMobile. However the savings really start when 5GB of LTE is consider - with the Internet search giant's plan coming in at $70 and telecom giant Verizon's costing $110 a month . Project Fi initially will only be sold to a narrow US audience that owns the Nexus 6, a smartphone that Motorola Mobility made with Google's help .
target: Project Fi will be hosted through Sprint Corp and T-Mobile's networks .
  It costs $20 for basic service and unused data is paid back to customer .
  The invitation-only service will work only on Nexus 6 phones in the US .
  Numbers will live in the cloud so users can talk on any connected tablet .