In general, you can run this MacBook Pro (and others) on very low wattage - because it has a battery. You do not need a 140W charger, or a 100W charger or anything like that.
However, there's a risk that you will drain the battery if your particular use of the laptop makes it consume more power than you can charge the battery with.
Depending on the particular model of the 16" MacBook Pro (2021) the maximum power consumption varies a lot. In practice, you'll see a power consumption of about ~5-15W when the laptop isn't doing much, and ~50-100W when you put it to work. If you really stress test the maxed out model while having the display set on max brightness, you will probably be able to reach a maximum power consumption of ~135W.
So in essence, for most people having a ~96W charger plugged into a Thunderbolt would be perfectly fine.
Answer from jksoegaard on Stack ExchangeVideos
In general, you can run this MacBook Pro (and others) on very low wattage - because it has a battery. You do not need a 140W charger, or a 100W charger or anything like that.
However, there's a risk that you will drain the battery if your particular use of the laptop makes it consume more power than you can charge the battery with.
Depending on the particular model of the 16" MacBook Pro (2021) the maximum power consumption varies a lot. In practice, you'll see a power consumption of about ~5-15W when the laptop isn't doing much, and ~50-100W when you put it to work. If you really stress test the maxed out model while having the display set on max brightness, you will probably be able to reach a maximum power consumption of ~135W.
So in essence, for most people having a ~96W charger plugged into a Thunderbolt would be perfectly fine.
I know it's late but just to add, I dock my Macbook Pro m1 Max 16inch and use an external monitor. Because the power consumption is low without an in-built monitor, I use a 33-watt GaN charger. I am using it for the last 1 year without any problem. Even I have used it while I am traveling as I don't want to carry a big and heavy Macbook charger. As long as the charger is a PD 3.1(Power Delivery) charger, it will work. If your Macbook is in sleep mode then you can even charge it using a PD mobile charger.
I swear I've googled this and been through 5 pages of results and haven't gotten anything except ads for specific chargers or pages about fast charging. Specifically, I'm looking for a car adapter to stick my MagSafe 3 cord into (so USB-C) but I need it this weekend and the only ones I can find at local shops are 45W. I don't need fast charging, I just need it to charge. Would that work or am I a bit screwed?