It's a big power bank basically.

Needing help on finding one that can be used with 250-300w heater that turns on for about 5 minutes at a time, since my car 12v outlet cannot handle it.

I see a few but some say no to heaters?....

not sure where to post, figure pc users might have some insight into this.

You're going to require a large unit to constantly power a heater.

Look into Bluetti

A unit you'll require won't come cheap.