I had the same issue as the other people in this thread. My iPhone 7 would drop about 3% an hour on standby. My last phone, a 6s Plus, had fantastic standby battery life and I'd get about 9 hours of usage. Went to the Apple Store, got a Genius who understood there was something definitely not right. He did a diagnostic on the battery and found nothing as I expected. I told him that I'd already reset the phone multiple times and put it into the DFU mode thingy which didn't fix it. He said he wanted to completely rule out software as being an issue and so he reset the phone and put some different version of iOS on it (I didn't really understand this, he said something about how our phones get the software from different servers or something like that). He had written down notes of my problem and told me to come in again if what he did didn't fix the issue and I'd get a replacement iPhone.
The issue was not fixed.
Five days later I went in again and got a different employee who I explained the issue to and how the last guy told me to come in again and they'd replace my phone. This guy didn't seem interested in what I was told five days ago and instead tried to school me on how iPhones work and that the standby battery drain was normal because the phones are "so powerful" he said. I sat patiently and listened to his spiel but then reminded him of what I was told by the last employee. He went off for about 10 minutes and came back to tell me that I would get a replacement iPhone because that's what I'd been promised. I got the feeling he wasn't happy and that he'd rather send me off thinking he'd educated me.
My replacement works perfectly and I now get similar battery life to my old 6s Plus.
This really shows that it all depends on who you get at the Genius Bar. If I had gotten the second guy on my first visit, I would still be stuck with a defective phone. Good luck, guys.
Glad you managed to get a non-affected phone. Do you mind sharing your results? I had the same battery drain issues and got an exchange last week at the Apple Store. I wasn't satisfied first and decided to do a DFU / restore with iTunes 2 days ago. I went to bed with 80% remaining and woke up 9 hours later with 76% (Wifi ON, BT OFF). Not bad at all, given that I would typically lose around 15% on my previous phone. Phone was set up as new, currently using default settings, haven't installed any apps yet other than LinkedIn and a few games.