We are trying to breastfeed but there are concerns from the doctor about baby's weight and we have decided to supplement with formula after every nursing session (doctor suggested pumping after each time but with how frequently baby nurses, I can't manage that and maintain my mental health).

I can boil water in the electric kettle to make it a bit cleaner from the tap. I want to pre-fill some bottles with water so we can just grab it, add powder, and go -- is there any reason we shouldn't just let the water sit in sealed/capped bottles after its been boiled?