Airplay and battery life

You're effectively doubling the amount of streaming that the device has to do. AirPlay 2 doesn't work the same way that the rest of Sonos does.

In a normal Sonos use, the controller tells the computer on the speaker where to go get the data from, and the speaker then does most of the work. The only data that the controller has to do is capture the information about what is being played on the speaker.

In AirPlay, the device you're using has to get the stream from the internet, and then send that stream to the speaker, adding an extra amount of streaming. Which will require extra battery power on whatever device you're using. The streaming is no longer being handed off to the speaker, but it's now actually going through whatever device you're controlling from. The only case in which it wouldn't be a double stream is if the content that you're playing is actually stored on the controller device.

I also suspect that transmitting a stream needs a lot more power than receiving one.

Could be, certainly. I don't have enough knowledge in that area to be sure 🙂

Just the layman sense that pushing out radio energy will take much more grunt than receiving it, a passive thing. This battery thing is why I don't like streaming from smart handheld devices; as it is they don't hold charge anywhere like my Nokia used to, for 3 days.