It just goes back to the battery to complete the circuit. Heat is created by resistance and if the energy going through the heater isn’t being resisted enough, there is no heat, just wasted energy. Most of these heaters regulate temperature by turning off/on/off and it’s the coming up to full resistance that is when the energy is “wasted” and doing it at human temperatures is a lot of off/on cycles. Also, it’s worth pointing out that not 100% of the energy is converted to heat, or you’d have a gigaton bomb instead of a heater.
