The limit is based on the combination of the home and the taxpayer. If married and lived in the same home then only one credit is available whether on a separate or joint return.
So in example situation 1, the taxpayer and spouse lived in one home in 2017 and one spouse claimed the energy credit for the home. The credit has been used for that home by the taxpayers.
For situation 2, the same concept applies. The taxpayer and spouse lived in the same home in 2017 and claimed the energy credit for that home, then the credit has been used for the taxpayers on that home.
If there were two homes involved then another credit is available for the second home.
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