These slippers are a European size 42. They heat up to a comfortable temperature in 10-15 minutes when plugged into my USB charger, which uses 5V and 900mA, a relatively low power draw. A 10000mAh power bank, for instance, would provide about 11 hours of heating. It's best to use a power bank if you want to move around while wearing them and still have them heated. I am very satisfied with them.

Minimal warmth