I understand that you wish to calculate how much you could deduct for electricity expenses related to the rental. It does not matter that it is unfinished space.
There are no strict rules on this as long as you can justify a reasonable method. If your tenant uses the garage say 50% of the time and the area of the rented garage is say 10% of the total area of your home, you could deduct 5% (50% of 10%) of your total electricity bill.
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