Various factors, but you should update your profile so that we know what phone you got. Usually standby drain is caused by a background process running. Unlike iOS (up until iOS9), Android does have true multitasking, which leaves processes running unless you do something about it. But this is one of many factors, without knowing device and firmware version you're on, no one can help.
My battery is also draining extremely quickly, even with power saving mode on. I know you can limit background usage on apps and have done that. I've also disabled apps that continuously run, but they enable themselves without me changing anything. I have the S6 edge. Any suggestions?