I use my jacket often for many hours so having a spare battery is necessary. I can tell my old battery does not hold a charge for quite as long so another spare may be in order.
I bike on fair weather days with temps down to freezing. The Power Bank to warms the pullover jersey I wear -- even on days that cold -- even when the wind chill makes the day colder. It's not necessary to use it at the highest (red) setting; white is enough to keep me toasty all 20 miles.
The size of the battery, but power and physical size, is a good choice. Honestly it's about the only battery that fits well in their clothes. It does what it needs to and it's easy to carry multiple batteries. The only problem I have is, they are still micro usb. Micro usb is very close to dead. USB C cables are everywhere. Please change the connection.