I tested the Standby mode of my launch Steam deck and compared it with some of my other handhelds.
Almost 10 days, which is awesome for a PC!
I was pleasantly surprised by my PS Vita with its 10-year old battery... 15 days, and it beat my Switch by half a day.
The battery of my 3DS XL was empty after 3.5 days.
Then I tested the battery life while downloading games.
Unfortunately, Steam-OS doesn't support turning off the display while downloading (so far).
But with minimal brightness the results aren't bad either.
My first run was without setting a bandwith limit. In 4.75 hours the Steam Deck downloaded 380 GB in total, so 80 GB per hour or 22 MB/s on average... almost the maximum of my internet connection.
On the second run, I set a bandwith limit of 20 Mbit/s (2.5 MB/s). Battery life increase to 5.5 hours, but the total transfer was only 50 GB.
My first run with the Switch was with 2.4 GHz Wi-Fi. It downloaded 55 GB within 7 hours, then the download stopped due to the energy mode, which activates at 20% battery charge.
My first run with the Switch was under optimal conditions: 5 GHz Wi-Fi and the distance between Switch and router was only two meters. It downloaded 130 GB within 4 hours, then the download stopped again due to the energy mode.
Essay Pro