From Pub 527:
How to divide expenses. If an expense is for
both rental use and personal use, such as mortgage interest or heat for the entire house, you
must divide the expense between rental use
and personal use. You can use any reasonable
method for dividing the expense. It may be reasonable to divide the cost of some items (for example, water) based on the number of people
using them. The two most common methods for
dividing an expense are (1) the number of
rooms in your home, and (2) the square footage
of your home.
Thanks
