Hi, our anti-virus needs to run in the background to keep you protected in real time - that does, of course, consume some battery.
Device manufacturers like OnePlus provide a modified operation system, whose part is often a "high power usage reminder". These flag almost every app running in the background. Of course you can close the app, but that will disable the protection, too.
What you can do is to allow the app to run in the background, more on
https://dontkillmyapp.com/oneplus, which is a nice summary of how various manufacturers use optimizing features.
However, if you still feel, that the app consumes more than it should, please report the fact directly from the app's Menu > Help & support > Avast direct help. That way the Support guys will get some additional technical details along with your report, and will be better able to help you.