In general for individuals (not companies) items of income, deduction, or credit must be reported or claimed on the tax form for the year in which the income was received or the expense paid. So you need to use that rule, unless you find an exception. For details on residential energy credits see https://www.irs.gov/instructions/i5695 , which has this to say about timing for credits related to your main home:
Main home.
Your main home is generally the home where you live most of the time. A temporary absence due to special circumstances, such as illness, education, business, military service, or vacation, won't change your main home.
Costs.
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. If less than 80% of the use of an item is for nonbusiness purposes, only that portion of the costs that is allocable to the nonbusiness use can be used to determine either credit.
If you were eligible to claim a credit in 2017 but didn't, in general you have 3 years from the date you filed the return (2017 due 4/15/2018 or later with an extension) to claim a refund. So you have time to amend if you do it soon. (time limit in 26 USC 6511(a) https://www.law.cornell.edu/uscode/text/26/6511 ) ...
Sometimes credits or deductions are limited, but the amounts not used in one year can carry forward to future years. I believe that can apply with residential energy credits if you didn't owe enough tax in the expense year. If you fill out the 2017 form with the energy expense if you should see that carry forward if it applies. But you can't just bring it forward to 2020. It would need to go to 2018 first (then 2019).
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