While in use, an Iron Sun box should be kept in your living space, close to where power will be used. All batteries, nickel iron included, perform best if protected from extreme heat and cold. Another benefit of keep the box in your living space is that it keeps wire runs to a minimum, which will also improve performance.
If you are storing your kit for emergency backup, it’s okay to store it in a shed or unheated building, but don’t leave it out in the rain.