Individual Data usage on WiFi network

But how would I see which device it is?

Are you suggesting that I only have one device connected to the WiFi per day and watch the meter online?  That's absurd.

@Hammspace This method is the best practice for seeing which device may have programs still running in the background. We are not asking for you to connect one device per day.

Another option is reaching out to our security team if you fee that the usage is being reported incorrectly. This link goes into how you can locate and report your concerns to our security team https://internetsecurity.xfinity.com/help/report-abuse

Another option outside changing the password to find the device that's using the most data is to look at the usage for each device if it's available.  I know with our iPhones we can see the usage for each social media apps etc. 

If you want I can look into unlimited usage and review your usage history with you to see if you would be a good candidate for adding unlimited to your account. This way you don't have to worry about going over the 1.2 terabytes of usage. 

If this is your first overage for the year it's covered through us. I have a link that goes into more details https://www.xfinity.com/learn/internet-service/data

Everything connected to the internet uses data such as our phones, tablets, smartwatches, smart tvs, any streaming movies, TV shows and our apps contribute to our data usage. 

If you would like to go over your usage please direct message me your first and last name along with your full service address. 
Here's the detailed steps to direct message us:
• Click "Sign In" if necessary
• Click the "Direct Message” icon (upper right corner of this page)
• Click the "New message" (pencil and paper) icon
• Type "Xfinity Support" in the to line and select "Xfinity Support" from the drop-down list
• Type your message in the text area near the bottom of the window
• Press Enter to send your message