If you plan to claim a deduction for Expenses for Business Use of Your Home (home office expense) on your tax return, you need a reliable method to calculate your deductible utility costs. IRS rules require that you separate utility expenses that apply to only the residential portions of your home (such as cooking gas or electricity used by your refrigerator) from those that pertain to the entire property. Only the latter type can qualify as home workspace expenses.
For example, if your cooking range, water heater and furnace all run on natural gas, you may need to figure out the gas cost associated specifically with heating your home. One way to do this is to average your gas bills from summer months when you did not use heat. This average shows how much of your gas bill is attributable to your range and water heater. By subtracting this amount from your gas bill for every month, you can calculate how much money you spent specifically on heat during the year. You may then be able to deduct a portion of this total as a home office utilities expense, based on the area of your workspace.
An experienced tax pro can give you other ideas for tracking and calculating the allowed utility costs associated with your home office. The IRS does not require perfect accuracy, but your calculation methods must be logical, reasonable and based on written evidence such as monthly utility bills.