Since it is more than allowed, you can use your best guess. Just keep in mind that, if questioned, you will need to produce a copy of the bill (perhaps you could get that from the company you purchased it from). Also, make sure your unit qualifies.
You can see which energy-saving home improvements qualify for a
federal tax credit at the Energy Star website.
The Residential Energy Efficient Property Credit is
what is called a "non-refundable" credit which means it
will only reduce your tax liability to zero. Any remainder can be
carried forward to the next year.
This link has more information about
the IRS Home
Energy Credits .
