Ok there is something you can try. The next time when standby and usage are same let the iPhone on so it will turn off when the battery is empty. Then go to: Settings--> Data protection (Datenschutz in Germany)--> Anlaysis (below) --> Anlaysis Data = and there search for "LowBatteryLog". There you will see what drains your battery. For me it is "mediaserverd: com.apple.audio.VAD". Search on Google for that and you will find some People with that error. That service is an Apple service thats seems buggy and not a app.
„...
apsd: APSCourier(tcpStream:dataReceived

NoIdleSleepAssertion == 255, held for 00:00:00
apsd: APSCourier(tcpStream:dataReceived

NoIdleSleepAssertion == 255, held for 00:00:00
assertiond: SpringBoard:59:59-5BFCC1EF-2F38-43C1-8091-B9549272DAC0 [FBSystemApp] [0x100b06010] SystemIsActive == 255, held for 00:00:03
mediaserverd: com.apple.audio.VAD [vdef] AggDev 44.context.preventuseridlesleep PreventUserIdleSystemSleep == 255, held for 05:32:56
mediaserverd: com.apple.audio.VAD [vzzz] AggDev 2.context.preventuseridlesleep PreventUserIdleSystemSleep == 255, held for 29:04:49
...“
So that prevents sleep/standby for 29h.... Usage and standby will be the same...