250W is crazily high and will burn you.
1W will do nothing.
So 10W is about right as a starting point for designing a warmer like this. Perhaps it should be 20W,
perhaps not, you need to experiment. Remember a lot of soldering irons use 40W or less…
I’m intrigued by the project and wanted to see what I could make and get a feel for what wattage a glove heater might need to be. I etched a heater from polyimide film using material that I requested for free from Dupont’s website:
The heater I made was this spiral thingie (see picture below) that I then cut up to be one long heater. It has a resistance of 3.5 ohms. It is just long enough to wrap around each of five fingers.
I’m powering it with an old weedeater battery that limits current to 1.5A. The battery is 20V so I’m using a buck converter to take it down to 6V.
To test I wore a thin cotton glove and then wrapped one finger with the heater and turned it on. I could feel it warm up quickly but didn’t get too hot. If I put this assembly inside of a winter glove I think it would work even better. At 9 watts it’s just right for my tastes.
A thought I had was to adhere the polyimide heater to the glove with thermal glue and then spray it with plasti-dip to give it a rubber coating and make it more water resistant. It would sort of be a hybrid of two different flexible heaters.