My understanding is that it's capacity is 8.8KWh and qualifies for $4500 federal. I'm definitely not an accountant though!
$4,502 is the tax-credit Prius Prime is eligible for. Federal Tax Credits for All-Electric and Plug-in Hybrid Vehicles
Form 8936 doesn't ask you to enter the kWh of the battery. So I'm guessing that you are trying to calculate the credit. What the 8936 instructions do say for line 4 is: "You can generally rely on the manufacturer’s (or domestic distributor’s) certification to the IRS of the credit allowable as explained above. Tentative credit amounts acknowledged by the IRS are available at http://www.irs.gov/Businesses/Qualified-Vehicles-Acquired-after-12-31-2009. Or you can visit IRS.gov and search for “Plug-In Electric Drive Vehicle Credit (IRC 30D).” Going to their odd looking link will yield the same magic number as the site that John posted a link for. By the way the formula (which I guess you know already) is 2,500 + 417 + (8.8 - 5)*417
