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same here, I tried various fixes mentioned, but still can't last the day, whereas I used to end around 20%. I gave up and bought a series 3, which has incredible battery life
 
My wife’s 38mm AW 0 has been fine on 4.1. She’s not complained about battery life since i installed it (the day it was released)

I just asked my wife and she has 43% left.

It’s 8pm and she’s had it on since about 6.30am.

I don’t think that’s bad for a 38mm series 0.
 
My 38mm Series 0 hasn’t had any issues with 4.1. I haven’t installed 4.2 yet. You might try updating or just waiting a few days to see if it returns to normal. Could be an app doing a lot of background syncing. Try removing Watch apps you don’t use, and make sure the ones you do keep are up to date.
 
Updated directly from the latest watchOS 3 to 4.1. Battery was very bad the first 2 days. Had to dig around with settings. So far I've switched the dock to Favourite apps only and kept only the apps that I use. Mail was using a lot of battery in the background so I removed it from dock and now battery life back to normal. It lasts full day from 8am to 10pm. If I go swimming with Swim.com app, I'm left with around 15% at 10pm. If I don't, I'm left with around 40%. I think it's not bad since I bought my 1st gen watch on the first week of sale. Other stuff that I do throughout the day: use Apple Pay several times, check Notifications, switch music that is playing on my iPhone, unlock my mac.
Overall I'm very pleased with the performance of 1st gen Apple Watch on watchOS 4
 
Updated directly from the latest watchOS 3 to 4.1. Battery was very bad the first 2 days. Had to dig around with settings. So far I've switched the dock to Favourite apps only and kept only the apps that I use. Mail was using a lot of battery in the background so I removed it from dock and now battery life back to normal. It lasts full day from 8am to 10pm. If I go swimming with Swim.com app, I'm left with around 15% at 10pm. If I don't, I'm left with around 40%. I think it's not bad since I bought my 1st gen watch on the first week of sale. Other stuff that I do throughout the day: use Apple Pay several times, check Notifications, switch music that is playing on my iPhone, unlock my mac.
Overall I'm very pleased with the performance of 1st gen Apple Watch on watchOS 4

You might try adding mail back. The first couple of days after a big upgrade like that there can be a lot of data syncing and indexing. I’ve sometimes seen terrible battery life a day or two after an update, but then it’s fine. Enabling mail may also have a temporary hit as it syncs for the first time. I don’t actually sync mail to the watch though, so maybe that’s why I get such good battery life. :)
 
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