I have been losing bytes at a high rate since I upgraded to gen5. Looks like it is my main computer that is taking them all - even when I'm not connected to a browser. Now that I've identified the culprit, how do I stop it?
In addition to Glasswire mentioned by maratsade, which can tell you what is using the data, make sure that, if it's a Windows 10 computer, you have "Updates from more than one place" turned OFF. You can find this setting under "Settings - Update & Security - Advanced Options - Choose how updates are delivered (at the bottom)". Turn Updates from more than one place OFF, if it is not already. Microsoft, in their infinite brilliance, decided that we should be able to share our updates with everyone on the net, and this setting being on can allow this to happen. I guess Microsoft would rather people use our data to supply updates to others instead of theirs.
Conversely, if you have more than one Windows 10 Computer, and they are networked, having "Updates from more than one place" on, but set to "PCs on my local network", can actually save you data, as they can share the Windows 10 updates between them, only having to download an update once, and then sharing it.
Also, using the cloud can chew through data, too, so if you have your computer set up to do that, it's best to turn it off. Syncing between your Windows computers (if you have more than one) can utilize your data, too.
One last thing, if you are absolutely sure that it's your main computer utilizing the data, this won't apply, but if you have a DirecTV receiver connected to Hughesnet, they can use a LOT of data, and there is no way to control it. This is in addition to the data that would be used for On Demand services, and they can use data even when they are turned off (it's actually standby). Because of this, connecting a DirecTV receiver to Hughesnet is normally advised against.