To enter the cost (including installation) of the solar powered ventilation system, click on Federal Taxes, then Deductions & Credits, then under the Your Home heading select Home Energy Credits and follow the prompts.
You'll continue past the screen for Energy-Saving Improvements until you reach the screen for Additional Energy-Efficient Improvements. The first choice in the grid will be Solar Electric Costs, which is where you'll enter the amount. This will carry the figure to IRS Form 5695, line 1, Qualified Solar Electric Property Costs.
This is a non-refundable credit equal to 30% of the cost of the property. Here's a link to the IRS Form 5695 instructions, which discuss all aspects of the various energy credits: https://www.irs.gov/pub/irs-pdf/i5695.pdf
