I've been using a 5w charger with my series 9 since new. Theoretically it should stress the battery less than the fast charger, since slower charging creates less heat.
Same for my 10, but not so much about less stress on the battery and more about that's what I've had bedside since my first Watch in 2019. Fast charging is not a must have for me as battery rarely gets so low that I need power quick.
You can use any Apple puck/cable with an USB charger and it will be just fine. I think the 30 watt USB-C is optimal but not required.
ADD: compatibility for the
OG charging puck lists all Apple Watches.
Apple
mentions, basically, any charger they sell these days. OR any 5W or more brick supporting USB-PD standard.
I've never seen any solid numbers from a reliable source mentioning how many watts are really being delivered during charging. Reminds me of when iPhone X came out and all the threads around here "why is my wireless charging taking so long?! I've got a 50W charger and it's slowwww!". Lots of guessing.
That said, guessing, like all USB-PD charging and Made for iPhone chargers, cables, etc, negotiation between device and charger and will never hit max the charger can do. In the case of the X, seem to recall wired would top out at about 13W and then taper off once 55-60% battery level was hit vs max wireless was 7.5W. In this
news story from then, anything above 18W didn't improve anything and even 18W was only marginally better if looking at bang for the buck (ie. already have a 12W iPad charger vs get yet another brick).
Qi2, which is MagSafe like, is 15W max. Guessing MS is same ballpark re watts. Qi is/was 7.5W.
