how do i deduct the cost of a solar attic fan?

A Solar Attic Fan will fall under "Solar Electric Costs" on TurboTax, which will flow to Form 5695 Line 1- Qualified Solar Electric Property Costs. See Images below for reference on where to enter it 

If You are using TurboTax Online

• From your Main screen Locate Deductions and Credits and then Home Energy Credits. If you don't see listed here, scroll down and click +add Tax Brakes. A menu will pop​ up. Locate the section Your Home, and add Home Energy Credits. This will add it to the main screen, Deductions and Credits Section as shown below.

• Answer Yes, to the screen shown below and then move forward to answer the next few questions until you reach "Enter any Residential Clean Energy Improvements"

• Enter the amount you paid for any property that uses solar energy to generate electricity for use in your home. Keep going through the questionnaires.

If you are Using TurboTax Desktop

• From the main screen, Under Federal Taxes, locate Deductions & Credits, then Your Home, and then Home Energy Credits. Click Start.

• Answer a few more screens until you reach "Did you have any of these Residential Clean Energy Costs in 2024?". Answer yes.

• Enter the amount you paid for any property that uses solar energy to generate electricity for use in your home. Keep going through the questionnaires.