I do not use my PC during off-peak times but still have a lot of data usage during off-peak time.
What is using my service ? I thought there would be zero usage at off-peak times.
Welcome to TPG Community!
We don't have any information if who or what device is using the service, but our system counts the data going in and out of our server accurately.
Aside from your PC, are there any other device connection to your modem either via wired or wireless?
If you left the devices connected via Wired and Wireless and they are On, but not being used, they will still consume data. It is possible that some background application are still running through them or they are set to auto-updates, etc.
Check your devices further or make sure that they are disconnected to your network.