You can't take split the cost between two years. It is one credit for one window installation. The windows are not yours until fully paid for. You need to amend 2019 and include the entire amount in 2020.
In 2018, 2019 and 2020, an individual may claim a credit for (1) 10 percent of the cost of qualified energy efficiency improvements and (2) the amount of the residential energy property expenditures paid or incurred by the taxpayer during the taxable year (subject to the overall credit limit of $500).
Under the Residential Energy Efficient Property Credit, you can get a federal tax credit for installing alternative energy equipment, such as solar electric property, solar water heaters, geothermal heat pumps, small wind turbines, and fuel cell property.
If you made energy efficient improvements to your home like energy-saving roofs, windows, skylights, and doors, you'll be able to claim the Nonbusiness Energy Property Credit for 10% of amounts paid for qualified energy efficiency improvements, up to a lifetime cap of $500 or fixed dollar amounts ranging from $50 to $300 for energy efficient property, including furnaces, boilers, biomass stoves, heat pumps, water heaters, central air conditioners, and circulating fans.
