Google Inc on Wednesday launched a new U.S. wireless service that switches between Wi-Fi and cellular networks to curb data use and keep phone bills low.

The service, Google's first entry into the wireless industry, will work only on the company's Nexus 6 phones and be hosted through Sprint Corp and T-Mobile's networks, Google said in a statement.

The service, called Project Fi, will automatically switch between the two networks and more than 1 million open, free Wi-Fi spots, depending on which signal is strongest.

The service will cost $20 a month plus $10 per gigabyte of data used. Customers will get money back for unused data.