Hangouts 2.1 arrived back in late-April. That particular update added the widget and improved the SMS experience. But while those two items were welcomed -- it seems there was also a side effect. We've seen quite a few complaints about battery life since users starting running Hangouts 2.1+.
As it turns out this issue is being addressed by Google. There isn't any sort of timeline for an update just yet, however a Google employee has acknowledged the issue in the Hangouts Google Group.
"The engineers believe they have found a fix! As soon as the fix is released I'll let you know! Unfortunately, I wasn't provided a timeline, so stay tuned!"
Basically, it seems they feel they found the cause and are currently doing some testing to verify the fix. Needless to say, that still leaves some Hangouts users with a less than ideal battery life. For now though, we can hope the fix has been found and when it rolls out -- it doesn't cause any further issues.
Having said that, Hangouts 2.1 was a pretty decent update overall. The widget meant quick access to recent conversations and the SMS bit was two part -- merged conversations and a simplified contact list. If for some reason you have yet to update to Hangouts v2.1 you may want to hold off till another update arrives, and otherwise, those who have updated can hope the battery fixing update arrives sooner rather than later.