It really shouldn't be draining when you are away or anything like that.
That aside, you can potentially look into Siri Shortcuts that are in iOS 12 to see if something there might be able to help out with something like that (although it likely still wouldn't be automatic).
Another setting to check is to make sure that your wifi setting for "Ask to Join Networks" is turned off. It would still automatically join a known network if one was in range. What kind of battery drain are you seeing? As mentioned previously, if you are seeing significant battery drain, there's a good chance that it is something else.