You can take the credit in the year 2020.
The IRS says,
"For purposes of both credits, costs are treated as being paid when the original installation of the item is completed, or, in the case of costs connected with the reconstruction of your home, when your original use of the reconstructed home begins. For purposes of the residential energy efficient property credit only, costs connected with the construction of a home are treated as being paid when your original use of the constructed home begins." See Instructions for form 5695.
Since the "Qualified solar electric property costs" are considered to be part of the Residential Energy Efficient Property Credit (see pages 1 and 2), then the phrase "costs connected with the construction of a home are treated as being paid when your original use of the constructed home begins" applies.
Since I imagine that the original "use" of your new home will not begin until 2020 (when the home is finished enough to move in), your credit can be taken in 2020, not 2019.
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