SAN FRANCISCO — Google is trying to shake up the wireless phone industry with a low-priced service designed to pressure major carriers into making it more affordable for people to get online and use Google’s services.

Project Fi debuted Wednesday, about two months after Google revealed its plans to expand its ever-growing empire into providing wireless connections for smartphones.

Google Inc. is selling the basic phone service for $20 a month and will charge customers only 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.

LESS EXPENSIVE PLAN

Project Fi initially will be sold to a narrow 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, before subtracting any potential credits for unused data.

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.

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.