I’ve used 5 volt 2.1 amp USB-A charger with a USB-C cable for M1 Max MacBook Pro 16.

I suspect anything 5 watt plus will work.

Here is an on topic YouTube video.

To be very clear, at less than 15 to 20 watts, your battery may drain faster than it can be charged if the computer isn’t idle.

This command tells you the current power draw:

sudo powermetrics -i 1000 -s cpu_power,gpu_power | \
    egrep '^Package Power:|^GPU Power:'

You will likely lose 5 percent of the charge efficiency. In my testing while typing this, my M1 Max used about 500 mW to 961 mW. That is about 1 watt. So a 5 watt iPhone charger would maintain my current power level. If you are using significantly more power, you will want a much higher power output like the 20 watt to 96 watt chargers.

@gidds also recommended a menu bar app to determine power draw.