Most likely source of data use is background processes on your PC

Date: Sun, 14 Dec 2014 12:12:55 -0500
From: Mike Fox
Subject: Wireless advice

in reply to Susan Turner, if I understand you correctly you are using a cellular modem (or as Verizon calls it “jetpack”) to provide wifi Internet for your house?

If so the most likely source of data use is background processes on your PC. Examples include Windows Updates, cloud synching (for example Dropbox or maybe your Android device doing whatever the Android equivalent of icloud is), automatic backup programs in either a PC or your phone or both.

Keep in mind that PCs and even mobile devices assume that if they are on wifi, bandwidth is unlimited. They don’t know that your wifi is actually backed by a cellular connection! Lots of software saves data intensive background tasks for when you are on wifi.

You could also have a virus or malware on your PC that is using lots of data.

When I had this problem Verizon was pretty helpful. I got tech support on the phone and they looked at my massive data use and told me it was coming from Dropbox.com which led me to realize I need to turn off Dropbox synch when on that specific wifi network. They even let me retroactively increase my data plan for that month to avoid overage charges. You may need to turn off Windows update for example. Try calling Verizon, ask for tech support (not billing!) and ask them to look at where most of the data is coming from. That will lead you to the solution.

Mike