Google Sheets has only been available to users with an Internet connection until now. Google has made it possible for users to work with or without a network connection introducing latest update to Sheets.
Google says “Whether you’re crunching big data or tracking your family budget, you don’t want to waste time waiting for files to load or re-doing edits that were lost because your Internet connection dropped. You can now get more done by switching to the new version of Google Sheets. It’s faster, supports larger spreadsheets, has a number of new features, and works offline.”
It also added “You shouldn't have to think about whether you have a WiFi connection when you want to work. So just like Google Docs and Slides, you can now make edits to Sheets offline. When you reconnect to the Internet, your edits will automatically sync. If you've edited Docs or Slides offline in the past, then you’re already set up to edit Sheets offline.”
Setup instructions are also available for setting up offline in Chrome.